Jon Gotow (via Michael Tsai):
Yes, the Open and Save dialogs keep appearing at their smallest possible sizes in Big Sur 11.1. It’s not just you, and it’s not something you’ve done wrong – it’s a bug in Big Sur.
Sadly, resizing the dialog so it’s larger only works on the current one. Every time you’re presented with an Open or Save dialog, it’ll be back to its uselessly small size again because Big Sur doesn’t remember the past size like it’s supposed to.
If this feels like deja vu, it might be because there was a similar bug in Yosemite where Open and Save dialogs grew by twenty-two pixels every time they were opened. Coincidentally, or perhaps not, Yosemite was the most recent major redesign of MacOS before Big Sur.