Wrestling Section: How many people would it need to make a decent roister?

The roster size doesn't matter, how the roster is booked and used is what matters. Even Impact! Wrestling's minuscule roster would work if they knew how to book talent and stopped ushering in **** like The American Wolves or Dollhouse.

I'd argue around 25 to 30 to keep things from getting tedious. You need at least 3 or 4 tag teams for it to be considered a division (8) maybe a minimum of 6 women to classify a division, at least 2 more if you're doing a women's tag division, which is pointless. A midcard of 5 minimum, undercards are pointless and unnecessary if we're talking 'enhancement talents', and I'd say a strong main event needs maybe 4 or 5 headliners? And that's just to keep stuff from being stale or tedious.

needs to be more than 3 in a division.

Depends...even 10 are enough if they know how to wrestle... B-)