I actually would be interested in hearing some of the more experienced voices in terms of arena size. But, I would say (especially given the time you have played the game) that an arena size of 6500-7000 would be standard. Considering your time in DIII, it probably should be even larger.
But every team is different. I personally have been building out my arena quickly while I am at a level where I don't have major transaction costs (i.e., I don't have to have big salaried coaches and I don't have to buy expensive Transfer List Players).