given the following design...(and that to the right of the mirror is the trim to a door)

1) I would prefer for a mirror to span the entire counter width, but then where would I put a towel hoop/holder? The tall cabinetry to the left I guess I could put a towel into the side of that (hate to screw holes into the side of it)? With a mirror the width of the counter, the light is easy to center and I think it looks better but not sure about outlet and towel placement.

OR

2) would you do a small mirror as designed, leaving a space on the wall to put a towel holder/rack, wall outlet, maybe a little shelf...if that is done - do you just put the light centered over the mirror or centered over the counter?

IMO, if you go with the smaller mirror as drawn, center the light over the mirror, your 'balance' will look a bit odd if you center the light over the space between the linen cabinet and door casing.

If you go with a full mirror...

1. you don't need its width to go linen cabinet all the way to the door casing. You can have it butt up to the linen cabinet, but you shouldn't have it go all the way to the casing. The edge at most should align with the wall of the base cabinet. Or you could pull the width of the mirror an equal amount off each and squeeze an outlet in on one side, left or right?
2. you could put an outlet on the side of the linen cabinet then box it in to meet Code on the interior of the cabinet.
3. if you insist on a towel ring (I am not a huge fan...), put it on the side of the linen cabinet, or on the right side of the base cabinet as long as it does not protrude that much to hinder the door opening? Putting holes in cabinetry is sometimes a 'necessary evil'.

that door opening is a double door and that half of the door is rarely opened, so the side idea might work.

would rather not cut up the cabinetry at all (outlet, towels, etc) hence why I am thinking on the mirror wall.

My personal preference would be more mirror to make it feel bigger and brighter. I'd put the towel holder and outlet in the cabinet as suggested as well.

This really all depends on what is more pleasing to your eye. I would never feel right in that bathroom because it would always feel off balance, so if I was given the parameters above, I would do whatever looked more balanced to the eye. It is tough though because no matter what you can get the sink, mirror, light and center of vanity in a line unless you opted for a larger 2 door base which kills a lot of drawer space.

I think the way you have it is the best way to go with the light being centered over the mirror. I would also put the towel rack in to the cabinet because having it on the wall next to the mirror would look weird and I also HATE putting holes in the wall, especially for things that get pulled on constantly. No matter how good the wall mounts are supposed to be, I dont trust them. The wood side of the shelving unit would be much stronger or you could just fold the towel and leave it on the counter, especially if it isn't a bathroom that isn't used all that often.

gonna discuss with my cabinet guy about making that wall thicker to support things being screwed into it. I do think the larger mirror makes for a better overall look. This is a master bathroom so a towel just laying there on the counter - what are you insane?

a 40" wide mirror I think would be a good fit there, and then one vanity light centered over the mirror.

Fair enough on the towel my wife would leave me if she knew I suggested that anyways. Guess the bachelor in me is still alive and well.

Not sure how much flexibility you have with the cabinets. Can you center the sink?

Towel ring on the cabinet thing to the left. Big mirror. Light centered on the whole opening, but use a wide light that spreads left/right most of the space, maybe a 4 bulb. Outlet goes in whatever is on the right side of the space. If that is no good, it goes on the back wall between the backsplash lip of the countertop and the bottom of the mirror. Put two in, if you need symmetry.

this is something that was considered...if you look at the dimensions above in the blueprint, the sink cabinet is only 24", leaving 18" for drawers.

Centering that sink cabinet (already about as small as it can be) just leaves little tiny drawers. More tiny drawers also equals more cost. I think I a leaning toward a larger mirror too, but your idea is a good one if I had more overall width to work with

goal with this was to maximize storage, since we have literally none. No linen storage, no towel racks - it's an odd bathroom we are making much more functional

I start digging into little details like where the outlet should go and it drives the mirror and light decision, etc. but I needed to start with function and storage as main drivers. That left side pantry/linen closet is a huge add. Currently it does not exist, it is just a simple base cabinet

outlet between counter backsplash and mirror...hmmmm trying to find a pic of how that would look.

This is something that oddly bugs me but whatever you do have lights on the outside of the mirror not just overhead. I remodeled my townhouse and had pendent lights come down from the ceiling but on either side of the mirror. This made a major difference in shaving. With your current design I would look at pendent lights coming down, I would make the mirror only covering the sink area, I would then do floating shelves in the open space between the mirror and cabinet. In your pullout drawer left of the sink, I would put the outlet in there.

The sink needs to be centered then the mirror and light bar go over it. Here is what I would do.

Increase the linen cabinet width to 21" which leaves about 36 for the sink cabinet. Cabinet with two doors and the bottom drawer would work in that space. You can put a mirror over the sink and leave room for a towel hook or ring as well as an outlet. This is less drawers which may even reduce cost.

Also the linen cabinet drawers will be more useful. A 15" wide cabinet has around a 11.5 inch wide drawer (inside measurement). Cabinet is 15 inches on the outside (13.5 inside) then you have drawer guides and the width of the drawer box itself.

Since this is a master bath, you probably want a nice faucet and many of these are wider. The centered sink and wider feel will make it feel more proportional.

Also, have the rings, bars, etc added to the plans or add them yourself. This allows you or your framer to add blocking so these are not all attached to drywall. When we built, I blocked for every hook, ring, and bar except one. I am now building a wooden shelf/towel ring to cover the hole where the ring was pulled out by my son.