Sorry for all of the stupid questions, but my search strings aren't turning up anything here or on google.   

What is the size of the internal hex that is used to hold the rear axle (25mm) on a double sided swingarm?

When I try to tighten the 30mm nut, the axle just spins.  I have searched the here and the interwebs trying to find what size hex (or tool) to use to hold the axle while tightening it back up, but can't find anything.

It's a 17mm.

You can try a double-nut on a bolt trick, or most moto shops have a multi-hex tool from MotionPro (IIRC) that'll do it.

Outstanding, thanks!