With an engine that turns in the normal direction (even when it's pointing backwards) you must use a special pusher prop as others have said. The range of sizes is much more limited than for normal props though. Some brands of engines have a special reverse rotation crankshaft which means you can use the normal type props put on what seems to be "backwards" on the engine. If you have an engine with a removeable front end like the earlier OS and Enyas then you simply undo the screws, turn the front housing so the carb points out the side by the exhaust and it then runs in reverse direction as well as it did the usual way. Well it does if you put the screws back in

There's a couple of cautions though. It's not a good idea to use a plain bearing engine as a pusher because all the prop thrust goes against the crankcase itself and the shim washer isn't designed for continuous use like that. With a reverse rotation engine there's far more chance of throwing a prop because the firing impulses are then trying to loosen the prop nut.