That is strange as I was testing this fro another thread (About over sized Lines but normal Text size)
I found that Messages > Preferences > Messages should the Upsize change I made straight away but not the downsize change - until, as you say you left the Messages pane and then come back to it.
However now I have doubled checked and all changes are only reflected in demonstration if you leave the pane and come back
The Size and Font Name are displayed instantly.
What does resize automatically is the text entry box in the Messages Chat window whether you are increase the size or decreasing it.
You are also correct in that the Incoming Override displays properly (and Instantly)
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously