What is the size of the Jam nut I need for the front control arm?

The nut you are calling a control arm jam nut, is neither a jam nut nor does it have anything to do with the control arm.

It is a castellated spindle nut, and is held in place by a cotter-pin, usually accompanied by a special washer.
Do not install and actual jam-nut in that location.

In a pinch
I suppose you could install any nut that fits, but you will have to figure out a way to keep it from unscrewing itself. but
Do not install and actual jam-nut in that location, as it will deform the threads. That spindle is relatively soft in that location.