It's not that large actually. I'd say somewhere between 200~300. It's one of the smaller cons, but honestly, I don't like *massive* cons, and most of those happen up North, which is out of my travel range seeing my budget and the price of trains and such. [That and I would go mad sitting on a train that long]
Edit: Apparently it's been refurbished since last year, and the new capacity is listed as '100-1200' probobly depending on the exact event. Given the stands and such, I'd change the upper estimate to maybe 400 at it's peak.
But that's 400 at the same time, people come and go, and there's 2 days of event.
From last year, at the start of the event it was quite quiet, but picked up during the day to being a lot more busy in the mid-afternoon [Peak feels like 3~6pm, events dependent] and then it would slowly die down.