Back in the 1950's, there were a number of free flights that were also for control line. The 30" (760mm) Goldberg Ranger 30 cabin used a Cox .020 Pee Wee for FF and .049 Babe Bee or Golden Bee for CL. http://www.outerzone.co.uk/plan_details.asp?ID=3830
You'll see photos of my Ranger 30 there as an FF. Berkeley scale kits had dual or triple designations, too. Some of those later became the Sig Craftsman series.
On all these in general, the engines used for CL were smaller than normally we are accustom to. There is a drawback, which is wind penetration. Calm weather permitting they did fine.
Regarding Scientific standardizing 18" (460mm) as the standard wing span, a half sheet of 36" balsa, in the 1950's and 1960's, kits were very cost competitive. These kits were sold in the thousands then. The market targeted school age boys. Older boys nearing adulthood and adults of course would opt for the larger and more expensive aircraft. The stand-off scale full fuselage looks of the Scientifics, with real decals and clear windows and canopies were very appealing. I was very excited when I got a Scientific Hellcat for my 11th Birthday. Kits then were under a couple bucks each. Also, you must remember that many of these kits were designed prior to the Cox more power .049 engines. The less powerful .049 OK Cubs are shown on many Scientific 1/2-A CL's.
Back then, there wasn't the stigma of a noisy unmuffled small engine in a school or public park like there is now. It started IMO shortly after the Vietnam War. People prior looked at it as healthy noise and that a child was occupied and staying out of trouble.
The RTF CL's IMO are also responsible for standardizing the 18" to 21" wingspans.
Although a novelty, personally I'm not into the so called scale flight speeds in control line. If I see a Fairchild Ranger cabin doing 5 second laps on 60 foot lines, even engaging a loop or inverted flight, I think it amazing and enjoyable. Control line is different enough that the fact it flies and flies well is enough to suit me.