What size fly is most commonly used when fishing lakes and rivers for bass? I've heard friends say everything from 8 to 1. Is there a way to simplify this without buying the whole store?

I assume you are asking about "hook size" ? I have no idea what the "most commonly used" fly would be, but can tell you what I like and prefer and use...

If fishing a new to me body of water for bass, I'd probably always start with a sparsely tied Clouser on a size 2 or size 4 hook. Second choice would be a bead head wolly bugger preferably tied on a long shank hook, again in size 2 or 4.

I'd also have one or two poppers/frogs for situation fishing.

Downsize if in a pond and/or small water body.

Good luck and for sure don't buy out the store. It isn't necessary at all to catch bass and have a lot of fun doing it.

This is great advice. Only thing I would add is that each fly represents a different portion of the water column. Poppers for surface, a bead chain clouser for the middle, and a cone head bugger to get down deep. Figure out where the bass are and you're covered, but that's always the million dollar question.

Cast to structure and don't be afraid of losing a fly or two and you'll have success. Good luck.

When I lived on a small lake near Tyler I did not know about Clouser's or Lefty's Deceivers (or half & Half) but had some Marabou Muddler Minnows in various colors such as Black, White, white and red. Strip and pause. Just my dos pesos, I am certainly not an expert.

Thanks for help!

Another factor is the weight of rod/line you are throwing. You can throw a weighted size 1 fly on a 7 or 8 weight rod but on a 4 weight rod it would be an exercise in frustration. 5 wt rod, stick with 4-8 size flies for bass. With a 7 or 8 weight rod you can throw bigger stuff up to size 1 or maybe even 1/0.
As mentioned above, don't go buy the whole store. Buy a few hat are recommended and seem to fit your needs, and go try 'em out.