It depends on what you want to run in Win 7. The installation itself takes up about 20GB. I'd give it at least 32GB plus whatever space you need for applications.
Note that Boot Camp uses the old base 1024 gigabyte instead of base 1000. Therefore, your 128GB SSD, which is really about 125GB because of overprovisioning, winds up being closer to 116GB in Boot Camp Assistant.