tl;dr - don't.
VMware Fusion has an 8GB limit, so on my 16GB MacBook Pro (which is an old one with a 16GB memory upgrade), I wanted to use more than 8GB.
You can fudge it my editing the .vmx file and changing the memsize value to whatever you want.
However it turns out that if you do this the performance on the whole machine turns into a pig. It beachballs all over the place, both in VMware and without. Constant hanging of VMware, fan running hot the whole time, etc.
This is just an apocryphal view, but having spent a week and a half with it unusable and me thinking I was going to have to go back to Win7, turn it down back to using 8GB exactly seems to be working much more stably.