Staff size and font names

[Basil Crow]

In the following sample:

 % change for other default global staff size.
 myStaffSize = #20

 #(define fonts
   (make-pango-font-tree "Minion Pro" "Myriad Pro" "Courier" (/ 
myStaffSize 20)))

what is the relevance of the "  myStaffSize = #20" line in relation to 
selecting font names. I don't quite understand how they go together.

Thanks,
Basil