I saw one at a LFS, that grew to 2 foot in a year, regardless of the tiny tank he was in.
A 8 footer could do for some time, but you'll have to be prepared for it to become BIG.
Def my favorite fish, but adult RTC can only be kept in the largest tanks, or ponds.
In my case, after seeing a 3 footer go berserk in a huge tank at the zoo, i decided that i couldn't possibly care for a 3 foot fish and give it the space that it needs..
And like Delboy said, a big RTC will LOVE Oscars......for dinner.
Second on my catfish wish list would be a Merodontotus Tigrinus or a Juruense catfish.
They both max at 2 feet (often not reaching that size) and couldt spend their whole life happily in a 8 footer.
those will not eat your Oscars!