Get the Large!
This bag is awesome! Not only does it have great little pockets for phone, money, tools and more...it has little pictures on each pocket to help you understand where those items go. It's reassuring to know that after my super long ride or my super harsh crash when my head may not be all there, I have extra help directing me to where my phone is located :) The large bag fits very nicely under my saddle and fits every tool, tube and extra you could need.
Get it, Enjoy it!