I would expect you to get a lot of responses with a pretty wide range of suggestions. 2 Oscars and a Texas Cichlid could 'survive' in a 55 gal tank for the duration of their life. They will be cramped, your nitrAtes will always be high (aka, water unclean, even if it 'looks' clean), but... they will survive (unhappily).
If you go with a 90 Gal, there will be more water to spread out the nitrAtes. Provided you do weekly 20-30% water changes you shouldn't have too much of a water quality issue. The fish will have enough turning room, you will have enough room in the aquarium to add sufficient cover while still leaving enough open swim area.
If you suit yourself with a 120+ Gal, you will be able to very happily house the 3 fish you mentioned, plus have room for a couple others. I read a post by Inchworm last night that suggested you get an aquarium twice as big as you need, because by the time the aquarium is stocked, you will need it. I could SOOOOO identify. Every aquarium I have ever bough is 'not quite big enough'.