Is this for your personal use or show? Our indoor is 60' x 80'. It is adequate, but we can't get a really good strong trot going in it for very long without having to slow down for the corner. If there were more than 3 turnouts in it, it would be too small. (Once the snow comes off, I can't wait to drive in our bigger arena outside!)
The arenas at the carriage shows around here are 100' x 300'. That is plenty big for the VSEs. The smallest size of a VSE Driven Dressage arena is 20 x 40 meters, or roughly 66' x 132'.
Myrna