It looks like the smallest you can get is 1.8" diagonal made for small phones etc and then you can get varying other sizes going up to about 15" but most places seem to want to sell a couple of thousand so they may be difficult to get hold of.
The DS touch screen is easy to get hold of and relatively cheap and you don't have to buy thousands 
Just do a search for a 4 wire resistive touch screen.
Mowcius