Size Matters! Medium or Large?

Like almost everyone else I encounter, I'm trying to decide on the right "size". Honestly I've never seen a Timbuk2 messenger bag so I have no idea. I'm in the military and ride my bicycle to work with this backpack that I hate. So I ask, medium or large? I keep reading that the large is massive but exactly what defines massive (dinosaur or gorilla)?

My daily requirements include tennis shoes, light rain jacket, towel, 2x t-shirt and undergarments, toiletries, shower shoes, phone and breakfast/lunch. Any advice would be greatly appreciated!
 
silly I’m unsure about the size of the messenger bags
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