Did he give you the manual? It should have some prop recommendations. In case you idn't get it, never fear. OS is really good about posting manuals on their web site. In fact, it's at this link: http://www.osengines.com/manuals/50s...1fx-manual.pdf

In case you haven't learned this yet, prop sizes tell you the diameter and pitch. The first number is diameter. The second number is the pitch. It tells you the number of inches the prop will move forward for each revolution. In general a prop with a smaller diameter but higher pitch will make a plane go faster than one with a larger diamater but smaller pitch. The reason I mention this is to suggest that as a beginner you want to choose a larger diameter, smaller pitch prop. This will be slower, but will actually pull the plane better. It's kind of like being in a lower gear on that 3 wheeler.