From my experience, sizing around A3-A2 is most popular, but also look at frames (e.g. ikea frames, wilkos etc) and look at the 'image dimensions' to use as a guide, as they are slightly different to standard A paper sizes and if possible you want to avoid trimming off the edges when framing.
For how much to charge it is always a difficult question. If you have a fairly large and loyal following, you can definitely get away with charging more. Also print quality would come into it. If you were to get giclee prints done, these could be sold for much more as the quality is incredible, and you'd want to make your money back.
I sell a few prints, but these are just printed digitally through instantprint. I get batches of 20 for about £15-20 (depends if there's a deal on) and then sell for £20 at A2 and £15 at a3 (postage is separate) - I think this is pretty cheap for prints, and definitely cheap enough that it doesn't scare people off, but for me the profit is still huge.
Maybe just get a small batch printed for now to test the waters. Then if they fly out the door then GREAT! If not, they can also be used in giveaways etc to boost your following, and you won't be out too much.