Hi Jeroen! Thanks for getting in touch–it’s awesome you’re checking out the WP Ecommerce Tickets plugin and I hope it’s a good fit for you. 🙂
Good question. We are looking into the idea of adding ticket meta to our ticketing framework. In fact, we started testing it this month and hope to have it out in a future version (though I’m not exactly sure which one it is planned for).
For the time being, though, I’m afraid that having a form inside of an event that works with checkout would take some custom development to accomplish. It’s possible you could integrate something like Gravity Forms and embed a form into the checkout screen where customers can select a shirt size. I think the heavy lifting will come in where that data is stored and if you want it to be associated to the event post.
Does this help answer your question? Please let me know.
Cheers!
Geoff