Extra Large
Those FF's are HUGE and have great turnover and rinsing. Martin is a GE FF pro, and he's right, they performn wonders.
Post some Model and Serial #s, folks here will help you find the filter pan and softener dispenser. If yours also has a mini-basket option, you gotta find one. Frankly, they don't thrill me, but others here find them very handy. And it is always good to locate all the parts and have the complete GE Filter-Flo machine.
Congratulations and enjoy!