Proper dressage and HJ arenas are usually twice as long as they are wide, while most western arenas are one and a half times as long as they are wide. english arenas also tend to be larger, probably because english riding emphasises speed and large movement, while Western riding is all aobut tight maneuvers and precision control. With this in mind, however, English arenas CAN be the smallest! (from the rulebooks)
here are some sample sizes:
western, starting with the smallest 'official' size:
80' x 120' (most common, esp. for public arenas)
100' x 150' (most pro shows are held in this size) (could this be because you can fit exactly 2 of em on a football feild
150' x 225'
200' x 300' (almost a pasture!)
and some sample English sizes:
50' x 100'
100' x 200'
150' x 300'
200' x 400'
250' x 500' (amazingly enough, there is a stable not far frim me that has an INDOOR arena this size....with attached stables, too, so indoor show horsies never have to go out in the cold - board at that place starts at $675 a month for bare bones)