Vicky, downloading PS2 games is piracy- no ifs, ands or buts about it. Also, the only official drive for the PS2 was 20 GB in size with the Final Fantasy XI package. If you are doing this on a 40 GB PS3 it would not do any good as the 40 GB system can't play PS2 games.
It is also a violation of the copyright agreement that you agree to when purchasing the game to make a copy- according to the agreement you are allowed to use the original disc but you are not allowed to make any copies.