Will a 128 gig solid state drive be big enough to comfortably run a dual boot of ubuntu and windows 7?
Yes, that's more than enough for both systems. Obviously, it depends on what other data you plan to have in these systems. But remember that Ubuntu can access Windows' file system, but Windows cannot access Ubuntu's. So if you're going to have other data on this SSD, such as video and music, you might want to allocate most space to Windows and then use that as storage for those things.
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