What criteria do you use in choosing an engine? Weight? Wingspan? Example: Just how big a gas engine could you put in an airplane with a 60" and say 7lbs? How do you choose a gas engine for say a 1.20 sized model? This is just a grey area to me that I would like some feedback from you experts. Thanks.

That's at the bottom end of gassers, unless it's a radial engine plane you probably couldn't get a gasser to fit or balance.

Usually they say a 1.60 can be replaced with a 26cc gasser, Saito has a 20cc gasser that might replace a 1.20 Nitro motor and a Chinese company makes a 17cc gasser, but you're very limited in the small size you're interested in.

usually, plane span and (single cylinder) engine stroke are at a fixed ratio of about 80:1

Rcgf just came out with a 15cc engine as well, it weighs about 29~30oz with ignition and muffler. At such a small size you are rather limited in your choices.

http://www.zjrcgf.com/productdetail.asp?id=103

Okay......I get the picture for the smaller aircraft, but when choosing for a larger aircraft, is there a rule of thumb for say ws vs engine or wt etc?

What type plane, Scale, Sport, Acrobatic?

The easiest way to go about it is... The gasser should throw the same size prop at the same rpm. If the rpm is lower on the gasser (usually is) then you need to throw a larger prop by at least one size.

It is also highly dependant on flying style. But the prop technique works well for all types. You also don't want to add to much weight to the airframe. If you are significantly adding weight to go with gas then the airframe will probably not fly very well (not true in all cases).

For me, gassers are really not suited for airframes below 9 lbs. (IMHO)