Looks like a Scat Cat without the profile canopy:
A .25-.28 is enough power for sport flying - see the engine range above.
Near as I can infer, that R528F engine is likely car/buggy derived, and high rpm but modest torque. I think if you try to prop it for max power up in the 25-30k range you'll have way too little disc area for this airframe. Maybe start around 8-6 and see what happens from there? I think you should be able to spin that in the 20k range maybe better? But I don't have an R528F so this is SWAG. A piped mediocre .32 is good for around 18k on that prop, whereas Jett .30's spin a
9-6 the same speed.
The .28 would be in it's glory in a smaller airframe, maybe like a QM15 racer, spinning the crap out of a 6" - 6.5" prop.