it depends on how big your hard disk is and how much you plan to use either OS. If you have a 500GB HDD 50/50 is probably a good idea. If you have a 240GB SSD or smaller (SSD's are awesome by the way) you might need to be a little bit more judicious about your choice of space allocated to each OS.
IT IS NOT EASY TO RESIZE A BOOTCAMP partition once you have installed Windows and all your apps on it. Resizing requires unofficial risky procedures which may end up causing you to lose data and reinstall Windows and all your apps again anyway.
There is an application available for a cost that apparently makes this procedure simpler and less risky. Paragon Camptune X. Though I have never used it.
I would not suggest using Windows backup from within Windows itself (unless you completely blow OS X off the machine and just have windows present), instead use Apple's included (and great) backup software Time Machine combined with an awesome program called Winclone: http/twocanoes.com/software.php?software=1
I have used Winclone and its awesome!
Also, I don't mean to make you regret your purchase etc. but now was a horrible time to buy a new MBA or MBP. Should have waited until a new one got released. Its always best to buy an apple product just after a line refresh because you either get the newer hardware for the same price or the hardware you would have gotten for cheaper. The worst time to buy is just before the lineup is expected to be refreshed because you basically end up getting ripped off.
As a matter of fact the next refresh should offer great benefits regarding battery life etc. since it will be making the jump to Intel's Ivy Bridge Processor.
Only use Windows 32-bit if you already own it. If you plan on buying Windows always grab the 64-bit version. There is little to no need to buy a 32-bit version anymore. Having said that, now is also a horrible time to buy Windows 7. Windows 8 is around the corner. Though if you need Windows 7 now its not that bad of a purchase.
I would definitely try to hold on on the MBA though for Ivy Bridge.