The ST90 will spin a 15x4 but don't expect stunning performance, especially if you plan to 3D that bird.

The ST90 is *not* a good 3D engine (lousy midrange/transition).

I'd also give the 14x4W a try (you'll probably get the same static thrust but more speed because of the higher RPMs) although my choice would be for a decent 14x6 wood prop.

Why wood?

Well, once you start getting up to 14" diameter and above, plastic props (like the APCs) get really heavy and that can affect the spool-up time of the engine (an important issue for a 2-stroke in a 3D plane). I've also found that some of the newer wood props (like Xoar) are actually quite a bit more efficient than the APCs.

If I were you I'd just go buy a bunch of props from 14x4 to 15x6 and try them out to see which is not only best for the engine but which is also best for your flying style.

I know that if I plan to do a lot of hovering, harriers etc a bigger low-pitched prop is good. However, once you get into maneuvers where you want to have a *lot* of very rapid tail authority (waterfalls, yak-offs, flips, etc) then a little less diameter and a little more pitch makes the rudder/elevator far more effective when necessary.