It must be a really powerful engine to run such big props. Just kidding, because OS calls out props that tend to overload the engine.
Whether that is a marketing ploy or a noise abatement strategy is unclear. But I doubt the laws of Physics has changed that much since the last time I ran a .75 sized engine. On a draggy biplane, I ran a 13-5 prop which would peak at around 12,000 on the ground. A 13-6 (depending on maker) is likely quite good too. If a faster design, then a 12-7 would be a good starting point or perhaps 12-8.

Now what nobody will tell you, but here is the straight dope. As you increase the load on an engine with an over-sized prop, your needle valve (high speed) get more sensitive to it's setting. Thus it becomes much harder to get a good setting that will overlook slight problems with tank position and changes in the conditions from the morning to afternoon. You also get a lot less power out of your engine, since the power is a combination of the rpm and the torque. Generally an engine will unload somewhat in the air, so you always prop 2 or 3 thousand below the point of peak power. For your AX 75, that is 2.37 hp at 15,000 rpm. So you don't want to peak on the ground above 12,000 or so. OS lists the practical rpm up to 16,000 so you would be fine at 12K.

Or you can load the engine with their recommended props and suffer with touchy needle valve and low power. If you need more power, well they make a 95 too.