If they are asking you to mail photos (rather than email images) I would send 4" x 6" photos. Some shows want 3 pics of products and 1 of your booth, some want 3 products, no booth, some 5 product, 2 different shots of your both, etc. And if you have individual pics it's easier to give each what they want. Be sure to label the back of each photo with your name, company name and email.
Also, you are applying to the organizers of local shows, in some cases they may use photos a visual aid to help them assign booth spaces