Depending on how much it's re-sized... it can look horrible or ok. It will degrade it making it larger, but if made a bit larger no one would notice.
From a professional standpoint? I make images larger all the time. Usually it's ok within reason. I wouldn't make a 100x100 pixel gif into a full page print ad though.

That just looks horrible.
Making images smaller degrades them yes... but I do that at work because they don't have ample server space for huge images. I keep images at 200 resolution (for newsprint) but make the inches smaller to fit my needs.

Smaller is ok provided clients don't come back doubling the size of their ad. Photoshop saves it as smaller so it would have to interpolate the larger version and Photoshop isn't very good at interpolating.
