There are a few things you need to take into account with props, especially on a quad.
1) Will the length of the prop fit on my quad?
Clearance can be a big problem if you're trying to use something that's not sized for your quad. You can always go a little smaller, but too long and suddenly it's hitting the frame when you arm your quad, and can break things, cut motor wires, hit antennas, all sorts of fun stuff that does damage.
2) what is the recommended prop size for my quad?
As mentioned before, check to see what is recommended for your quad's motors. Prop length and pitch both play into how responsive or how much power/speed you can get out of it, but if you put the wrong size prop on, it can burn things out really quickly. Been there, done that, not fun.
With those things in mind, let's talk really quick about the prop sizes. 6040 is a 6 inch long prop, with a pitch of 4. That essentially means that for 1 turn of the prop, it should move through the air 4 inches. You may see on airplane props that they're referred to as 6x4, but it essentially means the same thing. Quad props will usually have them listed in things like 5145, which would be a 5.1 inch long prop, with a a pitch of 4.5.
Where you need to be ESPECIALLY careful is that if you add a blade, the rule of thumb is that you need to go down in prop length, since you're adding more resistance to the motor with that extra blade. So, if they recommend a 6040 2 blade prop for your motor, it is recommended that you'd go down to a 5040 for a 3 blade prop. Here's the good news with that - 5040 and 5045 props are some of the most common sizes out there to find, so they're usually in stock and easy to find - if you're starting out flying quads, you'll want to stick with something easy to find, because you're GONNA break props. It's a given.
Looking at the Lumenier motors that you're looking at, you'll have no problems flying 5045 props, and it'll actually feel a little more docile, I think, which would be good for first flights.
