With the .55-AX, you'll likely want to experiment with 12" props like an APC 12x6 or 12x7. I've found that both work well on that engine. A 12" prop will mean very little ground clearance. You'll have to run bigger wheels or different landing gear such as what Dubro sells or the SSE gear from
www.tntlandinggear.com. Both will be more durable than the stock gear too.
With the stock gear, you can get away with an 11" prop. Maybe 11x7 or 11x8? I've never ran an 11" on my .55-AX but it might be worth experimenting with.
My favorite engine/prop combo on the SSE has to be a .46-AX or similar with an 11x5 APC. I'm sure the .55 will fly it every bit as good or better though.