Your primary website logo can be uploaded under Site Settings. This logo is also used as the default image in donation receipts and other sections of the control panel (e.g. in some email blast themes).
We recommend using 1200 x 400 (i.e. an aspect ratio of 3:1). That said, there is no hard limit or required proportions for these logos.
Logo variant for dark background
If you have a separate white version of your logo for use on dark backgrounds then you can upload it to the "files" section of your website homepage with a file name containing the phrase "light-logo". This logo variant will then automatically be used on pages with a featured content slider.