Here is 44 px height panel :
Here is 24 px height ( a quite frequent height for horizontal panels ):
…and 16 px height - which is the minimum :
As you can see on last 2 pictures, both vertical panel and firefox are overlapped by the horizontal top panel, which size actually never goes under a certain amount of pixels ( I’d say about 36px ).
Same with default theme and icons, pocillo ( first pictures are adapta and papirus ). And same without pixel-saver and global-menu.
It can be a bit annoying with some applications like scribus, inkscape, LOdraw where menus and rulers are heavily used.