FWIW, counterintuitive though it may be, thinner spokes make a stronger wheel because they tension correctly. So get the thinnest spokes that will serve the intended purpose.
10g (around 3.2mm IIRC) might be the right ones for a motorcycle rim (check the specs for yours from the place you bought from to be sure what size nipple holes it has), but what size are your spoke flange holes on the motor itself? If you have nothing else to measure with, then if you have allen wrenches of various sizes, you can see which one best fits in the holes, to at least give you a guesstimate. Otherwise you can check with QSMotors to see what the size is for the specific motor you bought.
To determine the actual length of spokes needed you'll need the ERD of your rim (from the domed nipple holes in it if you have to measure that, assuming it's not available from the place it was purchased from) and the spoke flange hole diameter and spoke flange width of your hub (should be available from QSMotors, if not you can measure these).
There are a number of spoke calculators out there; one interactive one is at http://ebikes.ca/tools/spokecalc.html ; places that sell motorcycle spokes may also have their own calculators you can use too. If you don't have one you use regularly in your wheel builds then you may want to use several to ensure all their results match so you can be certain of getting the right length for your wheel build.
If you have to drill rim holes at a different angle than they already are at, you will want to use washers that match the nipple size at their center and if possible are domed to fit well inside the nipple hole domes on the rim, for best nipple support and strongest wheel with least retensioning over time.
