Recommended partition size MacBook Pro (Mavericks) for Windows 8.1, Windows database app?
Windows 8.1 takes 20GB (16GB for x86), a database application like MSSQLS takes 1-5 GB depending on the features you install.  A database with 10,000,000 entries is roughly 1-10 GB.

150 GB is a lot, IMHO, but it depends what else you want to do with it and how much memory you have available of course.