There does seem to be a limit on iMessage length but no details about what it might be can be found.
Most likely you are going to lose this Message so grab a Screen Shot of it if you need to re-type it.
Use CMD + ALT + ESC to bring up the Force Quit window.
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