sleepimage file size
The sleep image file gets created to store the contents of RAM, so you should be able to make things fit by checking the math carefully.


How OS X and iOS report storage capacity  - http://support.apple.com/kb/TS2419


So, once you've made sure you don't have errors in powers of 2, you can make a small allowance for the filesystem overhead and try using a partition to store this one file. You will want to also consider that the OS will take control of this file on a very low level and perhaps cause the file to be written to the root filesystem even if you take pains to properly mount the auxiliary filesystem. Also, since swap files get written to this same directory, your plan might cause unintended consequences, so be sure you have a good backup and can recover any file should the machine run out of space for /private/var/vm and freeze out whatever app you have running.