The official system requirements for Windows 7 state that it requires 16 GB of space, or 20 GB for the 64-bit edition. Below that, it says that XP mode requires 15 GB more of space!
So, official specs say that a minimum of 35 GB are necessary, which would make affordable drives like this a no-go. That said, since 7 offers features for SSD like TRIM, and the pagefile can be moved to a physical hard drive, is it really necessary to have 35 GB of space? Bonus points if you have actual experience installing this kind of setup.