Can I preset the hibernation file size on a Mac so that it can always hibernate?
I don't think there is a way you can affect the size of the file /var/vm/sleepimage, which is the file created when the mac hibernates. You can move the location to which the file is saved to another drive instead of the boot volume using the command:

sudo pmset -a hibernatefile /Volumes/OtherDrive/sleepimage

This other drive can be an external HD or perhaps a USB3 thumb drive. Another option is to purchase a Transcend Jetdrive to use for the sleepimage file, or, better yet, move files from the internal SSD to the Jetdrive. 

I have doubled the capacity of my 128GB MBAir using a Transcend Jetdrive and it is more than fast enough for iTunes and iPhoto libraries.