I just measured the inside of mine recently to tile it and it measured ~ 30 3/4 x 12, I don't know about the height, I didn't bother with that, but I think it is 12.
Depends on the tile and the tank brand, as there can be slight variations. I have had 6" tiles that were actually 6.25", and tank dimensions vary slightly between manufactures. The best thing to do is measure the inside dimensions of your tank, then measure the tiles and have them cut to fit at the store if needed.
thanks man.. so i got the tank, it was used for saltwater, and seems to have some salt /calcium deposits in some spots.. do u think getting some CLR would do the trick?
CLR will work, just be sure to rinse very thoroughly after you use it. If you want to try a less toxic method first vinegar or lemon juice will often do the trick.