When you upload a logo to your screener, we re-size it to 300px by 80px for larger themes, or 110px by 80px for smaller themes BUT we display the logo at half those sizes so it looks good on retina and higher pixel density screens. So you upload at the higher resolution, and we size it down in the browser so it looks sharp on higher resolution screens. Lower resolution screens will also display it at a maximum of 150 pixels wide, or a maximum of 40 pixels high. If you have a design eye and don't want any server-side re-sizing happening, which we think is great, then pre-size to 300x80 or 110x80 for small themes.
Keep in mind different aspect ratios will result in different sizes - for example if your logo is 90x90 then it will appear in ethnio as 40x40.