From what I have read, and seen in most divisions, many managers will set prices to the max, and when these sell out they begin expansion. Its up to you and what you think your team will do this and next season.
If you plan on being in D.III for a few seasons, then I would suggest to wait until you sell out before expanding, and focus on playing with the prices.
If, however, you think you have a good chance of promoting, either this season or the next, then I would look at arena sizes for D.II and begin to construct yours to match it so that upon promotion you will have adequate income to compete at that higher level :)
Hope this answered your question
-NS