The logo size should be measured in pixels, but the dimensions should never be too large. Depending upon the desired size of your header, you should factor in an additional 30px that will be added through padding.
The file size should never really be more than 25Kb if possible. Remember that image file sizes add to the memory call upon your server and upon the visitor's browser, slowing down the deployment of your site on the visitor's screen.
Personally, my logo is wider and shorter in design, so I make mine less than 90px high. Since the header branding area takes the image size PLUS the margins and padding into account, you'll want to keep the height low otherwise you're pushing the branding 'below the break' and visitors won't see important things like menus, content, sidebars, etc.