I can give you an example of how i did on my macbook.
Macintosh HD has 95GB
BOOTCAMP has 25GB
In OS X i use most of my software. iLife, Final Cut Pro, Adobe Creative Suite, iTunes, iWork, so its gotta be somehow a big partition.
In Windows i use some work software, Flight Simulator 2004 and Office, so 25GB it's great.
To sum up, it depends on the software you'll be using in each OS. If you are willing to use a lot of games, or software that it's thirsty for disk space, then you should think how much space will it take. If you mostly use OSX, and you use software like Final Cut, Audacity or iMovie, then you'll need a lot of space for that to work correctly.
If you tell us a little bit more, like how much disk space you have, and what you plan to use in OSX and Windows, then we can give you some advice on how to handle the partitions.
Your 25GB won't cut it
Windows 7 SP1 needed 9GB free temp space
Hibernation - size of RAM
cache and temp files
the "core" of Windows 7 64-bit is 20GB alone.
I don't see the use of a 32-bit OS but that takes 16GB. And that is used.
If you cannot login - you will know why, or install updates.
I recommend minimum 48GB and even more for SSDs.
Paragon CampTuneX can resize
I don't have any issues with the 25GB. It works like a charm, even when i use many apps. Of course i installed the 32-bit version, since i have 1GB ram, 64-bit version would install, but i would be laggy. I have 8 GB free on windows partition and the page file is 2GB. I can login perfectly and never had any issues with updates.
Well most Macs today and last 3 yrs support and run better with 4GB (or more) and even Leopard 10.5 would find 1GB low, and even XP (though at one time 512MB was what was available, but that is how old XP is).
64-bit Home Premium and above, most people will find it USES 33GB and needs 20% and above free space for breathing and growing and that allows for a little room for apps and data. Not much. All my data is stored on 2nd drive.
So except for a very very few maybe 25GB but then Lion requires 2GB does it not? just for comparison.
if you have HDD less than 256 GB it will be hard to balance without one of the OS suffers, form my experience, with time windows is eating your space specially if you are a graphicanti/photograicanti thus at least 110 GB will be good for Windows, and for Mac OS not less that 90 GB this how it works for me.
It is not about how much RAM but I can point to 100+ threads when SL came out saying "my MacBook Pro; iMac, etc" was running snow very slow with 2GB.
Windows core code, is 21GB today and I have outlined how much more goes on top of that.
256GB SSDs are getting to less than $200. The question was storage. Old rule of thumb: DOUBLE your best guess.
I don't use very many p;rograms at all and none that big, and it shows 35GB but that does also not include some things I'm sure.
SP1 required 9GB minimum of free space. Temp but still has to be there. SSD needs 8-12% for TRIM and BGC to function (less shortens and wears more and much less efficient). Some SSDs have died when free space got too low.
On a Windows desktop you are free to install or move 3rd party apps to 2nd drive (laptops not an option usually) which also can be your data; even the default for /Users/ as well.
TimeMachine on notebook don't forget will try to store locally any changes if the TimeMachine drive is not mounted and available.... and use up a lot of space on your boot volume for OS X too.