why is the rom for alien vs predator 2 player when i recall it being a 3 player game in the arcades? i was hoping to find the 3 player one so i can play with my 3 gamepads ^^

You need to change the Test Menu settings to allow 3 players. I haven't tested this myself, so I'm not sure. FBA supports more than to players at least.

i just want to know how big of file this game is and what are the computer specs needed to run the game

Size: Not much
Specs: Seriously, No CPS-2 game requires obscene amounts of computer power. My Pentium 200 plays it fine.

Yes its been tested I've done this to all my multiplayer CPS games. I love it when me and my friends beat these games. For huge fun try King of Dragons on Very Hard only using 5 credits.