For whatever reason, X no longer automagically determines the dots/inch value for the right-hand portrait monitor and, for lack of anything better, defaults to 75 DPI. That scales everything down by 25% 33%, makes the menu fonts eye-crackingly small, and squinches the monospaced terminal font. The fix requires jamming the proper value directly into /etc/X11/xorg.conf along with all the other stuff:
Section "Monitor"
# HorizSync source: edid, VertRefresh source: edid
Identifier "Dell2005FP"
VendorName "Dell"
ModelName "DELL 2005FPW"
HorizSync 30.0 - 83.0
VertRefresh 56.0 - 75.0
Option "DPMS"
Option "DPI" "98x98"
EndSection
One could calculate the actual value, but that matches what X determines for the left-hand landscape monitor.
And then it Just Works…