We have 2 adults, and three small children, and we use a 50 gallon. Truthfully, just replace whatever size you had unless you used to run out of hot water. 40 gallon is another common size, and that may be plenty for you.
I replaced my first gas hot water heater about 6 months ago. Whole project took about 2 hours, and that included the trip to the plumbing store. It was very easy.
And as others said, go ahead and replace it. The water in the pan means there is a leak somewhere. Most likely the tank is corroded, and developed a crack. It make be fine for another week, or another 3 months, but at some point, it is going to burst and create a huge mess unless you replace it. Best to replace it now before it becomes a bigger problem.
Also, another tip. Use a sharpie, and write on the heater the date you install it (also, save the receipt). Most heaters have a 6 year warranty, and that date will make your life easier if it fails 5 years down the road. Mine failed with 2 weeks left on the warranty, and having that date plus the receipt saved me some cash.