MacOS BigSur

Do not use the menu > About This Mac > Storage panel to gauge your available storage. It fibs. Instead, click once on your Desktop startup drive icon (Macintosh HD), and press option+command+i to open a Get Info panel. The entry for Available is the figure that will determine if you have sufficient storage available for the upgrade.

Yes, at least 35.5 GB for install.

A few months ago I made a 36 GB test partition for Big Sur and could install to it (when booted from a flash installer). But (with that 11.0.1 clean install) the 11.1 update did not fit anymore. So I had to increase the partition to 64 GB (and at that time had to apply the full Big Sur installer because the update did not work no matter what, safe-boot included -- I did not try the disconnect from network trick though).

Matti Haveri wrote:

Yes, at least 35.5 GB for install.

A few months ago I made a 36 GB test partition for Big Sur and could install to it (when booted from a flash installer).

FWIW, the 35.5GB requirement is how much free space you have on a drive, not the size of the partition/volume you install it on.

Big Sur requires 35.5GBs of free space to download and install.