I'm trying to remember a movie/TV show where a guy goes into Victoria's Secret for a gift for his wife/girlfriend and acts really, really awkward. The man was relatively tall, a little chubby, but not too bad, I think. I thought it was Marshall from How I Met Your Mother, but I can't seem to find anything on it. I wish I could give more information, but that's honestly all I can remember.
Perhaps this scene from the UK sketch show Smack the Pony which stars the (rather tall) Darren Boyd? His embarrassment is played to the max, though he's suddenly very bold in a certain moment halfway through:
There's also a similar scene in Enemy of the State (available here), but I wouldn't call Will Smith chubby (and he's not that embarrassed). And finally, there's always this scene with Chevy Chase stumbling through countless double entendres while talking to a lingerie saleswoman in National Lampoon's Christmas Vacation: