I am a recent returnee to model aircraft, having been away from the hobby for many years. In returning, I decided to go with electric power, since I was building, & flying park fliers. Now that I moved up in size, I find that the cost of new motors, esc, and batteries to power these larger planes gets to be a bit prohibitive. I am considering switching to fuel and am looking for some guidance as to the best size engine to buy. My current thinking is to start with a 25 size. Is that a good choice, or should I go bigger? I have not found a good source for information about how to match up engine size to aircraft size.

You match the engines to the plane size but we tend to go a bit bigger on an engine that the plane or kit call for. Most kits and ARFs state 40 size or 60, 90, 1.20 and that gives you the main clue. You don't want to buy an engine before the plane, a .25 on a 40 size plane isn't going to work. After you get the plane you may need to decide on a four stroke or a two stroke engine.
May be a good idea to go out to a club or field and see what the other people are using and the type of planes they are flying before you go out and buy anything. It's a good idea to find someone to help you out with your choices, a good instructor is a big help. Not one of the guys from the peanut gallery, a good instructor.

Thinking 40 - 60 size is a better way to go. More of a selection in terms of planes.

Every kit or ARF I have seen lists a range of recommended engines, usually a 2 stroke range and a 4 stroke range. Some people like to use the minimum recommended size and fly their models similar to the way many full scale planes fly (even if their model is not a scale model). Others like to use the maximum recommended size (or bigger) and fly their planes in ways most full scale planes could never fly. Neither preference is right or wrong, you should just do what is fun for you.

A lot of people pick the plane they want and then choose an engine that is appropriate, based on their preferences.

My preference in airplane size is models with wing span between 55 and 72 inches. This usually makes them "40 size" or "60 size."
My 40 size models get .70 four strokes (maximum 4S recommendation for a lot of 40 size models) and my 60 size get .91 four strokes (except one that has a .61 two stroke).

The .40 and .60 sized planes are the most popular because they are the best compromise between visibility/ flyability and cost/portability. In making the switch, you also have to consider the cost of your field equipment and fuel, not to mention the extra wear on your covering from residue. I love glow planes, but the electric guys do have the more convenient end of the hobby.

Thanks for all the good input. Here is what I have in hand, and want to fit an engine too. It is a high wing, 52"ws all balsa and ply constr. covered with econocoat. It is home built based on an RC flyer pull out plan of a 40"ws electric. I did build that version and was pleased with the result. Wanting a larger version, I up scaled the plan and built the new one. As an electric, it has a auw of 65 oz. and dose fly well. The motor that I installed, is a little small though, and looking at the cost involved to refit all the electronics makes me stop and evaluate. What I want to do is convert to fuel, and have no idea how to size, hense this thread. I'm looking for some sort of formula to equate plane size to engine size.

The Sig LT-25 is a high wing trainer that is supposed to weigh about 4.25 pounds
Recommended two stroke engine size for it is .25 - .32

I would probably choose something similar for the model you described.

Two stroke engines from .25 to .36 are usually very close in overall size and weight. In many cases they fit the same engine mount with no modifications.

Another possibility would be something like the OS .46LA. It is bigger than the .25 class engines but is about the same weight. It is considered somewhat weak in the .40 group but one of our club members says he thinks it is a perfect match for a trainer like the LT-25.

I would go with the LA .46 also. I have a few of them and they arean't as under powered as most people seem to think, just not as hot as the better FX and AX engines. The price is right too!!