Tweak Me v2 - What size should my header image be?

There is no specific required size for the header image, because it depends on your Tweak Me theme settings.

Width
The width depends on your "Layout Width" size in Appearance > Theme Settings > Layouts.

Whatever value you enter in this box is the maximum size of your header area. So in the above example, you may want to aim for your header image to be 1100 pixels wide.

Height
There are no height limits/settings for the header image, because the header area will expand to accommodate any height image.