There are a number of issues that need to be taken into consideration, like image content and quality and whether it's commercially salable. But first of all, don't spend too much money getting prepared because it takes years to figure out how your work will sell in different markets or locations. Given a choice, people spend the least they can to get away with for a size that fits their space. 16x20 prints should be matted and framed 22x28 for full frame 35 proportion and 24x28 for full 16x20 proportion. All of mine are matted and framed to 22x28 to make up my display. Most of what I sell though is 8x10 matted 11x14 in unframed browse bins.

Some resources:
My photography forum made up of 3700 photographers discussing art show issues. A great place to ask questions.
The resource web site for my forum with over 100 resource for doing art shows.

Response to Monte to bring it back to the top.

Thanks Larry. I can already see it takes some real thought to what will sell and what size works. The people I have talked to say the Art showw sell more 11x14 and 8x10 prints. mpst are not framed or matted. Some larger prints do well if content is of real interest. I have years of learning what works. Monte.

You can't sell unmatted prints at an art show. They need to be matted to a standard frame size and displayed in clear archival bags or shrink wrapped (the bags are by far the easiest) so people can handle them.

I am sorry Larry I did mean prints in bags with mounting boards. I have some at the Art Gallery here and that is how I display them with the bags and foam bord behind them. Monte.