Bootcamp Assistant won't let me go over 20GB for the installation partition

Hi, everyone!

I'm trying to install Windows 7 Professional 64-bit on my mid-2012 11" MacBook Air and I've been having a problem with my Bootcamp Assistant in which it will not allow me to increase the size of the Bootcamp partition over 20GB. It only seems to give me two options: a) the default 16GB and b) 20GB. I do have 65GB available on my local storage and I would like to allocate more than 20GB to Bootcamp, preferably 40GB, but this just won't happen. I recall having read something about this problem a long time ago but I've forgotten what it was and what I have to do to rectify this. If anyone can give me some advice then that would be great! I've tried looking this up in Google and all I seem to find are threads about people wanting to resize the Bootcamp partition after they've done the installation. Sadly enough, I've not even gotten to that point yet.

Thanks in advance to contributors.

Regards,

Aek