Hi @Wolf2024 

Welcome to the Sonos Community!

The size of the screws doesn’t seem to be mentioned anywhere, but the Era 300 guide mentions drilling 9/64” (3.5mm) holes for the wall-mount’s screws. 

I hope this helps.

Below is a screenshot of the installation instructions for the Sanus speaker stand for Era 300.  They use an M5 screw, but the length will obviously depend on your particular application.

Ah - I think I may have misunderstood! Thanks @melvimbe!

Well, the Sonos stand and wall mount documentation only refers to the them as ‘short screws’…

Note that the bottom of the Era 300s are not flat, so if you’re using a custom bottom plate it either needs to be a rod like the Sonos stand, covered at the correct angle like the Sonos wall mount, or set on the two ‘feet’ at the bottom of the speaker, which is probably around 7 inches apart (depth) given the speaker dimensions.  And of course, with a flat surface, you’ll need longer screws.