is there an open source license that says says free to use for people etc.. but if you have a company larger than x then you must support the project with $$$ ?
I don't know of any projects that do exactly this, but there are projects that are dual-licensed. You can either use the code under the GPL or buy a commercial license and use it in your proprietary code.
ExtJS is a great example of this sort of dual-licensing, and I've seen it on other projects as well.
Anything's possible, but caution advised to anyone who tries this. It's not always apparent to open-source developers how much pain custom or obscure licenses cause in big organisations (ie the kind of paying customers in your proposal).
Even if it's just a normal license with a single clause variation, it becomes a different license from a tracking perspective. A lot of companies have standard policies on open-source and anything not following one of several popular licenses might be forbidden or require a drawn-out review process at best. This gets even more complicated when you consider packages might not be directly used, but be part of a dependency chain where you have to map out the whole graph in order to demonstrate compliance.