Required space for dual boot

The Ubuntu installation requires between 3-4 GB hard drive space, and 8-10 GB will be needed to run comfortably. But you may need at least 25-30 GB day to day.

Windows 7 on the other hand needs 16GB of available hard disk space (32-bit) or 20GB (64-bit), but that's just for windows itself.

Keep in mind that Ubuntu once installed, will have an office suite, utilities, and a lot of other applications, where with windows you need to install all these apps, which will require additional hard drive space. Just to give you n example, Microsoft office pro fully installed requires 3.5-5GB.

^{Sources:Ubuntu & Microsoft}