I have a covered breezeway from my backdoor to the garage (connects from house to garage). It is about 15 ft long and 6 ft wide. There are currently metal posts, but I don't like them and want to replace them with wood. My questions is what size posts do i need to use as replacements?
Also, I will likely attached fencing & gate to them on the driveway side.

Have you considered boxing metal posts with 1x cedar?

Yeah I would just box them. Removing them is A) going to be hard and B) going to shorten the lifespan of the structure.

Structurally you probably wouldn't need more than a 4x4.

Aesthetically, you may want something larger.

Ensure your connections at the ground and roof are strong, especially if attaching a gate and fence.

but with a gate and fence, I would want to set a post next to the breeze way post with as small a space as possible and that way the fence is not tied to the breeze way and can move independently. If you're intentional with what you are going to do before you do it, you should be able to make the 'boxing' close that gap even more but still leave a gap for movement. That way over the years it still looks good when it does move a bit.

What size or the metal post? 2 inch ?

Can you post a picture of the existing set up?

They are 2 inch metal posts. I dont believe they are required for structural support of the breezeway, so hopefully I can screw them into the concrete at bottom and breezeway cover above