If that's all you're doing 40GB is certainly plenty. If anything, it's a little much if you aren't going to be using any of the above heavily and might have use for the space on your Mac partition for media files or whatever.
Windows itself takes up a couple of gigs or so, AutoCAD is around a gig, Inventor is 4GB or so, and depending on the game you're looking at somewhere between 1 and 5 gigs in general. Even 20GB would probably cut it, certainly 30GB.
This is again if you're trying to minimize the size of the Windows partition; if you don't have any dire need for the space, 40GB should give you plenty of headroom, and is probably a good number.