So i just found out that my chainwheel on my old french bike is bent... i am considering changing it out but am concerned if i buy another righthand chainwheel and crank arm combo that it won't fit due to the fact this its french and the diameter of the spindle might be some odd size that will only accept a french made crank arm... is there sizing differences in the diameter of the spindle for older french bicycles?

Bicycle Mechanics would've been a better place for this question.

Cottered or cotterless? If you are just replacing the crank, pretty much any old square taper should work (if it's cotterless.) Sheldon Brown has an article on the JIS vs. ISO tapers, and IIRC his conclusion was that it doesn't really matter that much.

its a cottered crank

In that case, I would consider this a good time to ditch the cottered parts and get an aluminum square taper crankset and spindle (you can use your original French threaded cups.)

I think French bikes used different sized cotter pins, and I am not sure if that matters as to what crank you can use. Sheldon Brown has a good article on French bike interchangeability: https://sheldonbrown.com/velos.html

Yes, french had a different cotter size. +1 good time to upgrade. There are several choices of new french bbs for cotterless cranks. Prices are higher than british standard, but not bad given these are parts made for bikes out of production over 20 years. Of course, some french bikes had Swiss bb (more expensive and fewer choices), and eventually they all transitioned to british standard. Some french brands loved Swiss bb threading (Motobecane comes to mind...)

So it depends on what threading you have. Double check before buying anything.

If you have a bike shop in town with boxes of old bits, you can probably find a used square-taper spindle that fits your existing cups and bearings, probably $5. I just did that on an old Peugeot, saved the $50 for the Velo Orange French thread cartridge square taper bottom bracket. Or, you can splurge for the VO part.

Thanks for all the suggestions about how to switch the crankset out and the info about different size cotters ... maybe i wasnt being clear enough... if i wanted to buy an old raleigh heron crankset or maybe some old japanese skyway crankset would it fit on my french spindle? Does the size of spindle diameter differ with french spindles (axels) as well? It seems the measurements of the hole in the crank arm that the spindle fits into are not available. I checked sheldon brown and couldn't find much info there. I guess i can just buy some old right side chainwheel and cotter arm sets and see if it fits.. if not i guess i can just sell it back

The wall thickness of some old euro cups is thinner than that of Japanese cups. In all likelihood, a Japanese spindle will not work with the extant cups; usually this combo results in the adjustable bottom bracket cup threading so far into the frame you cannot fasten the lockring. Only way to find out is to try. You might be okay using the Raleigh spindle and crank.