By default, Boot Camp will reserve 20 GB for your Windows partition and leave the rest for a single Mac OS X partition. You can see this on the partition slider in the Boot Camp Assistant immediately following your download of the Windows support software. If you drag the grey dot between the two partitions on the slider, you can adjust how the hard drive is split between the Mac OS X and Windows.
Before you commit to your slider selection, consider the size of Windows itself and how you'll be using it. Windows 7 requires 16 GB of drive space for the 32-bit version and 20 GB for the 64-bit version. That means the default 20 GB is a tight squeeze for installing Windows 7, and you'll probably want at least 5 to 10 GB more for small software apps and files you'll use exclusively in Windows.
There's a good chance you'll be using Windows to run some specific software. If so, you can add that software's requirements to your total. For example, if you're using Windows to play "World of Warcraft," add at least another 25 GB of space to meet the game's minimum requirements, and you'll probably want more to save game data and install expansions over time.
As you grow your Windows partition, though, don't ignore the space you'll need on the Mac side. The slider should show you the available space you have on the Mac side as you increase or decrease the amount on the Windows side. Boot Camp Assistant will not let you go below 8 GB of available space on the Mac side, but you'll probably want a lot more than that if you use your Mac for storing or editing music and videos.
When you're happy with the slider position, click "Install" to continue. If you haven't installed your Windows disc, Boot Camp Assistant will prompt you to do so. Next, we'll navigate your Windows install and setup.