Everything is really small on high-density screens
I've got a chance to test briar-desktop on an ultra HD screen (3820x2160px on a 14 inch display) on Linux.
Apparently, it's common that the content of apps looks really tiny on such screens, I think we know the issue very well from Android, where it's solved nicely. On XFCE here I do have an option in Settings → Appearance → Settings → Window Scaling where I can select 2x instead of 1x and most apps look good. Our app does not, not sure what we need to do to pick up this setting.
This screenshot makes this visible: