Hi,
Hope you've solved your problem. If you've not, maybe this will help. I'm using OS 10.7.3 on a MacBook Pro with an Epson 2450.
I received the same error this morning, and tried unchecking the custom size box. Didn't work.
Since I repair permissions weekly, didn't try that thread.
What I did do - that worked - was to restart, and zap the Parameter RAM. You do that by holding down command-option-P-R immediately after the restart 'bong.'
I usually let it bong 3x more (takes several seconds for each), and then let the normal restart occur.
Good luck.