Well the wow folder is up to about 8G now isnt it? It might be even more then that. Then the OS will be like 1G or so, then you need like 1/2G or so for each instance.

The 5 files in the Data folder are 7G themselves, but I bet 95 percent of the reading is those 5 files.

16G would really cover it well but thats not divisible by 3 I guess 18G is the sweet spot.