I'm picturing this "fieldhouse" to look similar to the Kibbie Dome in Idaho or the Fargodome in ND only on a larger scale.
The Kibbie Dome holds 16,000 and the Fargodome holds 18,700. I'd imagine the Flames are looking at 25k max. The stadiums in Hamilton/Ottawa hold 24k, Molson Stadium in MTL is at around 25k with the renos. Mid 20's seems to be the trend in the league.
I'm expecting the fieldhouse to be a disappointment for the Stamps and their fans but tremendous for the other sports who will get to use it year round. In the end, the city just doesn't support the team well enough anymore to justify building a huge, beautiful stadium so we'll end up with something new but not necessarily overly nice. And I think many Calgarians will be just fine with that.