The key here is to understand pitch speed. The pitch of a prop is how far it will advance in one rotation if there's no slip, so a 5" pitch prop spinning at 15,000rpm will advance 75,000 inches per minute, or about 70mph. That means that when your plane reaches 70mph, the prop cannot produce any thrust.
Now, exactly how much thrust is required to reach any particular speed is very hard to estimate, and how thrust drops off with speed is also hard to estimate, but it's safe to say that the above plane will have a top (level) speed of under 70mph.
The first step is to look at the recommended prop for your motor. Often it will say something like this:
Use on 2s with 9×4.7 slow fly or the GWS 9×5 direct drive propellers.
Can also be used on 3s with 8×4 or 7×3.5 propellers.
Here you can see how increasing battery voltage spins the prop faster and requires a smaller prop to keep the power within the motor's limits.
A larger, lower-pitch prop (or a smaller, high-pitch prop) will have roughly the same power requirements. The normal rule of thumb is to add an inch of diameter when you reduce the pitch by an inch, or the other way around. So if a motor can handle an 8x4, it could also handle 9x3, 7x5 or 6x6.
The larger prop will have more thrust, but the smaller, higher-pitch prop will have a higher pitch speed, and thus a higher maximum possible speed. Prop pitch is a lot like gears on a car of bicycle. Too low and the top speed will be very limited, too high and it'll be hard to get going.
Here you need to go back to rules-of-thumb. Props are made in a range of sizes that are useful for most models. Generally the highest pitch option is the same as the diameter (a so-called 'square prop') and is only useful to very fast, sleek models like pylon racers. Models with more drag will need more thrust, and won't need the high pitch speed.
For that motor, you'd use the following props on these types of model:
6x6 - pylon racer.
7x5 - 'sports' aerobatics or flying wing
8x4 - trainer
9x3 - 3D aerobatics (i.e. prop-hanging) or slow 'vintage' flight.
You can use the same proportions for other motors: a 'square' prop for speed, diameter = 2 * pitch for a trainer, etc.
If you're not sure, order a prop either side of what you think the ideal will be and see which you prefer. Props are relatively cheap and you'll probably need replacements at some point.
Ideally you should also test a new motor/prop combination on the bench with a Watt-Meter, so check it's not pulling more current than recommended (or the ESC can handle) but you can also just check how warm the motor gets when run for 30 seconds or a minute. Warm is fine, but if the motor gets hot, you'll have to be careful in the air until you're sure the extra airflow is enough to keep it cool.
