If thats the only leak your c 4 has your lucky!  I had mine rebuilt by a transmission shop and after a year or so it started leaking and I never really was able to get them pinpointed and stopped.  What reading I did on the subject said that was typical for a c-4.  that doesn't make it right !!

I cant be much help with the size but I doubt an oversized one will go into place.  best thing is to take the old one off and take it when and where you can find them and get one as close as you can.  then when you start to insert it to the tranny put a little tranny fluid on the o ring so it dont roll off or bind

if your seals and lines are good then your c4 shouldn't leak.  Any of your local part sources or transmission shops can get you the proper Oring with out any pain and it should be a buck or two at the most.  make sure your trans if cool when you swap that oring or you'll have fluid all over.  Do it in the am after the car sits all night.  easy to do if you can get under the car. 

Don't let the c4 scare you.  Its easy to work on.

It has been suggested by some other sources that some well placed silicone sealant be used sparingly in conjunction with the o-ring.  Should I?