Minimum is probbly 60GB for Windows 7 64-bit, 80GB is safe, and when using SSD you want to allow 11% to be setaside for provisioning and almost a month's writes space.
Because Windows will on notebooks default to larger page size than you need, and hibernation, take your 16GB and double it and realize you want to take that into account.
Windows in theory is about 22GB but it is only after the install and updates - updates are highly compressed and needed to be unpacked and temp files written during an install.
Yes, Mac can get along and use just 40GB even.
Games and large apps can use more space.
356GB is an odd number for any SSD. 500GB yes, 480GB too, must be 120GB x 3 somehow.
If you are using 80GB for mac, then I would go for 50/50 or about 160GB for each. Won't hurt and won['t have to change it later.