EF,
The A,B,C, and D refer to old designations based on engine size. To the best of my recollection A went up to .15 cubic inch displacement (2.5cc) B went to .29 (5cc) not sure about C but D went to .60 or maybe .65 (10cc)

Go with the 50# line connectors regardless.

I have never used Kevlar.

Regarding line sizes, go to www.modelaircraft.org and look for competition regulations. Then go for control line stunt--precision aerobatics--and there will be information that is very good for sport models. Generally up to .40 engine lines are 0.015 inches diameter and usually 52.5 to 70 feet length. Above .40 stunt generally uses 0.018 diameter. There are also rules for scale models under that category that cover heavier models. It is also broken down to whether you are using 2 or 3 lines. Usually the same line lengths as stunt. The FAI scale class has a maximum of 21.5 meters IIRC which is slightly over 70 feet and a minimum length of 15 meters.

I have a hard copy of old scale rules-the website may be different because we have had a snarled up rules change within the last year. The rules talk about single strand lines, but scale fliers don't use them. Multi-strand, like the Sullivans, call for 0.021" diameter for two line systems for models up to 8 pounds and 0.018 for three line systems. If model is greater than 8#, 0.027" is specified for two line systems.

Our rules are silent on the size of insulated lines, but I think everyone goes by the size of the stainless wire core. I doubt the insulation provides any significant strength. Most of us here use coated fishing leader. The stuff advertised as approximately 120# test is the 0.027" steel cable. I use this with two 15# models and also with a 7# model. There does not seem to be a significant adverse effect on the smaller model. I only use one insulated and one bare line with my electronic systems. Industrial supply houses here, for example www.mcmaster.com sell wire rope that is what many of us use for lines.

Maybe someone else will comment on smallest practical model for electronic control. My smallest two have a .61 or two .30 engines.
Chuck