(I sent this to the other thread but my post never appeared there for some reason)
I don't know the format of the arxiv tex files but if you have the source you can either change the document size to a5paper in the document class declaration or if the template doesn't support it, use something like this:
Afterwards you might need to crop your pdf. I found some instructions in this post
but I've never tried to do that.
If you want non-standard sizes then the geometry package is the way to go.