How do I get a summary of all of the user's choices?

for example, I want to be able to know how many size M and size L shirts to order without looking at each submission individually and tallying up the results mannually

What I would recommend you do is create a Visual Report for your form.

Here are guides that will help you in creating a Visual Report for your form:

https://www.jotform.com/help/187-How-to-Create-a-Visual-Report-with-Your-Form-Submissions

https://www.jotform.com/help/281-How-to-Use-Visual-Report-Filters

I hope this helps. If you have other questions or concerns, please feel free to contact us again anytime.

Thank you.