Short answer: Depends on what kind of performance you want.

The first method would be to use your experience, and start with what has worked well for you in the past. Of course, you can use other people's experience as well, by looking at similar weight and type aircraft, and seeing what those models use.

The second method would be to determine several engine's thrust output, and make your choice based on a thrust-to-weight ratio. (At least 70 percent thrust-to weight would be good, I think.)

The third method is something I remember from Andy Lennon's book, and that is to aim for approximately a 240 to 250 oz. per cubic inch power loading.
Your plane, at 26 pounds (416 ounces) then would call for an engine of about 1.66 cubic inches. (416/250=1.664) This is only a general guidline, used for "sport" flying. yo may want more power if you're looking for extreme performance.

Hope this helps.