Bobby_M on this site has done an analysis of the grain bill / OG you can brew in various mash tun sizes here: http://www.homebrewtalk.com/f11/how-big-your-mash-tun-needs-123585/
But that's for the more traditional cooler MLT AG method.
I realize we're talking about BIAB here, but it would make sense to me that if you can make an average (OG 1.059) beer in a 5-gallon cooler, you could conceivably do it in a 7.5-gallon kettle. Someone correct me if I'm wrong, because I'm just guessing ... the mechanics of the BIAB lauter/sparge process are completely foreign to me.
@william_shakes_beer: If you can't do AG, you may consider moving to a partial mash just as a stepping stone for now. It does give you a lot more control than even extract + steeping.