I am currently trying to convert a Red Max GZ25N weed eater to a rc engine. Its displacement is 25.4cc or 1.55 ci. It runs well and has a walbro carbuerator on it. I was wondering on the size of propeller to put on it. I think it should be between 16x8 and 18x8, but i am not sure. Also if anyone knows about what size fuel tank i should use for it, since i will be using it for sport flying. If anyone could give some ideas here it would be helpful. thanks.

I would say the 16x8 should be about right. After you get it running, you can try a few other props to see how they do. Use a tach to check the RPMs.
I would try a 16 oz tank first, it ought to give you a fairly good fly time of around 10 minutes or so. Maybe a 24 oz tank if you like to fly longer. The carbs usually have a fuel pump built in so longer fuel lines and larger fuel tanks should not be a issue.

The Red Max is made by Zenoah, and the engine is very close to the G-26. So, if you use G-26 specs (props, rpm, break-in schedule, etc.), for reference, you should come out ok.

Does anyone know of any conversion kits for this model.The most necessary part would be the prop adapter.

IIRC, the diameter and taper of that engine's shaft is the same as a Honda. You can most likely get the adapter here: http://www.carrprecision.com/Pages/prod02.htm

Wacker engines has conversion parts too.
http://wackerengines.com/

so, Geoffinpdx, the prop hub and adapter from the Honda GX25 should fit on the REd Max flywheel?????????? It looks like the flywheel has three instead of two holes to mount the adapter to.

I was only considering the flywheel fit on the shaft. If you go with aftermarket ignition it'll make the conversion simpler, and you can get variable timing to boot. I'd call the two businesses and get their advice.

Geoff