Russell,

The 12.25x3.75 is the ultimate 3-D prop.

Your plane will not do "classical" aerobatics very well with this prop.
When flying in level flight, it will look like it is 'running full throttle in first gear'.

In other words, the RPM will be disproportionate to the speed.

The Bolly 11.5x6, from Adelaide, will give you good speed AND excellent vertical performance.

A 10x8, as was suggested, is the correct load and will give you still higher speed, but vertical performance will be badly hurt.