Hi jpk -
Sailors call that the "leeward" or "low" side of the boat, and the side toward the wind is called the "windward" or "high" side. It sounds like you need to get aft or back further into the high side corner. The straps across your trampoline are for "hiking out." Slip your feet and shins under that strap, scoot your butt all the way out off the edge of the windward hull, waaaaay back so that your practically sitting with your thigh under the steering crossbar. Now when you're accelerating, lean back, using the straps to keep you on the boat like you're doing a sit-up or ab crunches. This will help keep that leeward bow from digging in and ending the fun!
On the question of bigger sails, I'd recommend you get to a point where you're not capsizing before going bigger - the boat's already pretty powerful with the main it has. Going with a bigger sail will make it tougher to control and you'd be likely to flip more.
Keep pushing it!