Unfortunately, the only source of information on these motors that I know of, Hobby Lobby, does not recommend a prop for this particular motor/gearbox combo in their description. There's nothing like navigating uncharted waters...

Unless you want to get a Whattmeter and a bunch of props, your only choice is to find someone who has been down this road already. This means you're going to have to "pound some pavement," and poke around in the search engine on this site, other sites, and perhaps even Google...