I'm familiar with some of what you have but not all of it. About the Saito, is that an 80 or 82? If it's an 82 I would try an APC 14x4w, if it's an 80 I would try an APC 13x4w. I'm not too sure about the ST 61. It would be good to end up with more of a 3D type blade for the U Can Do but you may have to experiment with different ones to see what works best with it. If you fly at a club maybe there are people that will let you try one of their props to see what works good. That's the problem with prop / engine combos, you might have to try several different ones to find out what works best. So if you can borrow one and try it out that would help a lot.
About the prop length question, what it will do is either allow the engine to throttle up faster or throttle up slower or slow the plane down. Generally if the prop is on the small side the engine will spin up quick but won't pull the plane through the air as efficiently as one that's longer. A prop that's too long will bog the engine down and can slow the plane down.