face-font-selection-order

face-font-selection-order is a variable defined in `faces.el'.
Its value is

(:width :height :weight :slant)

Documentation:
A list specifying how face font selection chooses fonts.
Each of the four symbols `:width', `:height', `:weight', and `:slant'
must appear once in the list, and the list must not contain any other
elements. Font selection first tries to find a best matching font
for those face attributes that appear before in the list. For
example, if `:slant' appears before `:height', font selection first
tries to find a font with a suitable slant, even if this results in
a font height that isn't optimal.

You can customize this variable.