Washer Fluid Reservoir - cap size?

No clue what size it is but you can still buy the actual part for a few bucks online so I'd just do that...
 
Well crap. It was not long ago that I could still find Mopars (J3223202) and I planned on ordering one or two. Never did. The only option is Crown, same part number, not so great reviews.

For me personally, not too big of an issue. I've been running a reservoir from AutoZone since 2013 because my stocker was dead when I bought the Jeep. The cap that came with my reservoir works perfectly on my reservoir so no problems here. That might be a solution for you if your tank is in rough shape generally. I think it was like $40 so not too bad.

Since I can't get you a good cap replacement option, I measured mine to answer your original question. My cap has a circular opening of about 1.75" and the neck is slightly larger by probably 1/8" to provide the nice tight "pop" when closing it.