The barn I lesson at has an indoor arena that is 60 by 124 feet... Yes, it is big enough, and during the dead of winter you can fit 5-6 riders in there (EXTREMELY cramped!) There are 24 boarders there and 13 lesson horses, so the indoor gets lots of use.
It is big enough for dressage, we have the letters and everything in our arena. The trainers use it a lot for training some greenies, so I'm sure it would work. I would say go for it!
Intermediate Rider~Learning Equitation and English Pleasure~Interested in Eventing
A horse will tell you everything you need to know about him, if you listen.