Yeah if you had the standard hermits the colors are quite obvious.
Try looking on the net to see if you can get an idea for the species
http://www.liveaquaria.com/product/s...cfm?pCatId=501
http://www.marinedepotlive.com/inverts--crabs.html
With out an ID it hard to guess what size they will get, But id say 2" is the average size.