I am setting up a new Invision board, I want to know what's the best format/size logo I should design that goes with the default theme for the Invision Community forum? I couldn't find that information in the guides. Thanks!

The logo can be just about any reasonable size. The header size expands as needed to display it. You're constrained somewhat by width, so you don't overlap the other menus.

There's also a setting in the Theme itself to set the size as a fixed percentage.

Great. Thank you Mark.

You're welcome! Glad we could get your question answered.