[branding] Document intended settings for window size

- Control Calamares window size through the branding file
 - This is for #1062 and #1038

src/branding/default/branding.desc

@@ -14,6 +14,21 @@ welcomeStyleCalamares:   false
 # may have surprising effects on HiDPI monitors).
 welcomeExpandingLogo:   true
 
+# Size and expansion policy for Calamares.
+#  - "normal" or unset, expand as needed, use *windowSize*
+#  - "fullscreen", start as large as possible, ignore *windowSize*
+#  - "noexpand", never expand, use *windowSize*
+windowExpanding:    normal
+
+# Size of Calamares window, expressed as w,h. Both w and h
+# may be either pixels (suffix px) or font-units (suffix em).
+#   e.g.    "800px,600px"
+#           "60em,480px"
+# This setting is ignored if "fullscreen" is selected for
+# *windowExpanding*, above. If not set, use constants defined
+# in CalamaresUtilsGui, 800x520.
+windowSize: 800px,520px
+
 # These are strings shown to the user in the user interface.
 # There is no provision for translating them -- since they
 # are names, the string is included as-is.