Replying to both posts of your K3 - Yes.

For a fun jet or flying wing, the prop can actually limit you more than a trainer plane would. Most funjets are in the 4-6" range, whereas some trainers could fit a 9-14" prop on the front. A lot of people choose to put the largest prop on an electric conversion as possible, but I prefer to use what is "normal" OpJose is right though too, you don't need to start this way, but when you get past the W/lb estimation, there are still a lot of factors to narrow down and I like to choose a prop size and go from there. You can always dither back and forth from your starting prop size to find what you prefer.

Going back to the second to the last post you put up, that is a good way of doing it, but like Opjose said, let the numbers show you what ESC size you need. I'm glad you had some motors to pick from, so I could find one that might be in a good range.

Like most people will say here even regarding nitro planes, buy a bunch of props and see which one makes the plane fly the way you like it to.

It is near impossible to say how the plane will act once in the air, because, depending on the actuall aerodynamic effects of the airframe, it may unload differently in the air than on the ground. Try Try Try and make sure to get yourself some kind of Wattmeter.
Regards,
Curtis