If you are going to get into electrics, you need to buy a Wattmeter. The WattsUp meter is great, that's what I have.

If you do some research on your motor, you can usually find some data on the current draw for various props. HobbyCity usually has recommended prop sizes and/or user posts with prop size & current draw data.

Props for small electrics are cheap ($2-$3), so get at least 2 or 3 different sizes and try them out.

Different prop types and brands will cause the amp draw to vary, even among props of the same size. In general, APC e-props draw less than MAS e-props of the same size (APC props are more efficient, less drag). APC e-props will also have less current draw than APC slow-fly props of the same size. So if someone quotes prop sizes and current draw for various props on specific motor, make sure to note the brand/type of prop they used as well. For example, I found an APC 10x7e prop draws about the same as an MAS 10x6 electric prop on one of my motors.