From the AWWA On-line catalog:
"AWWA C500-02: Metal-Seated Gate Valves for Water Supply Service
This standard covers iron-body, bronze-mounted, nonrising-stem (NRS) gate valves including tapping gate valves, 3 in. (75 mm) NPS through 48 in. (1200 mm) NPS..."
AWWA C509 just covers up through 20". I think these are the only AWWA valve standards. So while you can get larger valves, they won't be "AWWA" valves.
From a practical standpoint, this is probably the point at which you're moving from a cast-body stock item to a welded-steel custom item.
And do check the prices on other valve styles. On a 10" valve, there's not a lot of difference between a BFV and a gate valve, but the larger the valve, the more the difference. The physical size gets to be an issue as well. A 48" gate valve is about 10' high.