I have several different custom print sizes that I have added over the years. Some are for one printer, while others are for a dye-sub printer for which there are many different sized prints for different products, etc. Right now I have 58 print sizes and need to add several more. Is there any way to divide print sizes into separate files in order to make them more manageable? Say one for each printer?

Fred interpretted your question to be about media sizes but the way you worded it, it sounds to me like you are talking about print sizes (in the print size list).  So I'll give answer #2 in case you are talking about the latter.

If you right click in the list of print sizes, there is an option on that right click menu for "Load" print sizes and also one to save print sizes.  So you could make two lists and save them under different names and then just right click and select "Load" to get the sizes you want whenever you need them.  If you've already added a few dozen print sizes, you've already been using that menu because it's the same menu where you create a new custom print size.

Regards,
Mike