That's why Adam's answer was correct to begin with. The partition has to be formatted for the install. If it's done beforehand, then Windows 7/8 CP will go on & install. If not, and the user has to format with the install disk, whs's answer applies. 100MB for W7, 350MB for W8.
Unless the user plans on using BitLocker, that 100/350MB partition is not needed or necessary. It's only taking away 1 of the 3 primaries allowed. Actually, it's 4 allowed, but in that case, no data partition can be made.
And speaking of BitLocker, it's not the best app that MS has produced. It can be buggy at times. TrueCrypt is a far superior app for encryption, and can be used on many OS's, not just Windows 7/Vista/8 CP Ultimate. And I've never had trouble opening a TrueCrypt volume. Can't say that for BitLocker.