What are you taking into the office?
A laptop. Various papers. I do worry that it might be a little big
Get the smallest, most practical thing that will hold your stuff. Whatever computer bag is designed for your size laptop will be fine. Get one that has the extra pockets or whatever you'd want for tablets, cords, the size of files you'll be dealing with.
You're smart enough to know what will look ridiculous in your office setting, i.e. backpacks are great and I usually use mine on personal trips, but I'm not rolling into court with it.