There are two settings at
Admin → Settings → Required
logo url
The logo image at the top left of your site, should be a wide rectangle shape. If left blank site title text will be shown.
reset
logo small url
The small logo image at the top left of your site, should be a square shape, seen when scrolling down. If left blank a home glyph will be shown.
I guess you could try using the same image for both, if that’s what you mean by “default size”
Otherwise I think you would be better off using an image app to make a compatible logo.
Using the same logo for “scrolled view” could make things a bit cramped there.
