i have heard or read of two specific instances where female ball pythons became egg bound. One was with a het. lav female and the other was a normal (possibly a gravid import). With the lav.. well, PM me and i'll see if i can find you the story. The gravid import i talked to a guy who was taking her off of another guys hands in hopes to rehabilitate her. She'd been gravid for 2 months or so past her lay date but looked to be in good condition. I dont have any update on that story.

I think its more of a spontaneous occurrence rather than "at x size it happens" if there is a large egg or a blocked duct it may be more likely to happen.