Default logo size
The default logo width is set to 165 on installation, but that leaves the logo truncated.
On a clean and brand new Wordpress 3.12, it looks like the attached: the Wordpress administrator toolbar doesn’t display
I’m happy to wipe the database and use the demo data as detailed at http://www.simplethemes.com/blog/entry/theme-content-generator/ but it lists only Onyx and Impacto
Uploading and trying Evenflow as an alternative theme on the same clean install displays the admin toolbar OK but its logo looks truncated too.
On activating each theme, I already went to ThemeName Options and hit Save to activate the settings, and tried using the Reset [to defaults] too.
Someone had the same question earlier this week. I put together a screencast that should help.
As for the logo size, It looks like the dimensions just need to be changed. Appearance → Breeze Options → Appearance. Obviously they’ll need to be changed for your new logo as well. I’ll put it on my list to take care of on next update.