35mm beyond 14x17

Hello,

This is a subjective question, mostly. I have an art show next week
and wanted to blow up a negative to a pretty large size, long print
about 20x40, for the fascade of my booth. The staff at the print shop
said beyond 14x17, it doesn't look very nice.

I imagine that the grain gets really large, and the color dots start
separating? I dunno, maybe it looks kind of cool, and the fact that
it's over three feet long overpowers any negative reaction from
graininess?

I'd just try it for the hell of it but a print that large costs me
over 120 dollars, so I'd like some feedback first.

Any and all opinions, thanks!