I started at 10.3 and moved to 10.5 and it is still a little nose heavy. I would not be afraid to start at 10.5

With a Tower .75 the wing loading is high enough to allow you to generate some pretty good speeds.

If you are after a sport flyer go with the OS .55 lots of good reports using this engine. If you are after a little more speed go with the .61 or .75. I don't know much about the .91 performance.