What is the minimum partition size for Mac OS 10.10 Yosemite?

I'm about to set up a Mac mini to dual boot Mavericks and Yosemite for software development purposes. What's the recommended minimum size of a Yosemite partition?

I have run Mavericks/Yosemite on a 64 GB SD card and after a clean install, there was about 20-30 GB free.

I would say that you could install Yosemite on a 20 GB partition, but I would not recommend anything less than 30 GB.

Instead of rebooting constantly to test software in different environments, try a virtual machine. I use VMware Fusion. Major advantages include suspend rather than shut down, and you can have as many as you have disk space for. You can have as many running simultaneously as you have memory for, and you don't need extra hardware to handle different versions - the hardware is simulated, so it's even possible to run future versions on older hardware within some limits.

My VM library includes 10.6 thru 10.10, Windows XP thru 11, Ubuntu, and DOS.

It is possible, but unadvisable, to run OS X 10.10 Mavericks on a 16 GB partition. I am doing so at the moment during a reinstall.

Mac OS X prefers to have applications on the boot drive, as well as the user home locations, swapfile, and sleep image file.

if you are not willing to hack files, then a 32gb partition is a minimal install partition for practical purposes, allowing for the roughly 12 GB system, 8 GB of vm files, 8 GB of sleep image, and some apps.