I couldn't respond right away because I needed two hands to pick my jaw up off the floor.
Have people no manners?
I mean, sure it's a compliment that they like your quilt enough to want a huge one, but, if you had painted an 8x10 they liked, would they dare ask you to add more canvas and paint it bigger?
I agree: say you don't do commission quilts. That settles it. That answer goes across the board so you don't have to worry if someone else will ask you to make one for them. (Unless, of course, you DO want to do a commission quilt?)
I've had people ask me to hem their kids' athletic shorts. I just say no, I don't do alterations for other people. (For me, I don't have the confidence--wouldn't want to screw up a pair and end up owing them a new one.) They say, "Oh, okay. I'll take it to a tailor." It wasn't hard once I started stating it as my policy.