We want to put in another sink/cabinet in the bathroom... a small one. OUrs is short, and the one I was thinking of is about five inchest taller and a bit narrower.
Will this be a problem or must I buy the same size. I'm headed to Lowe's and Home Depot.

Should be no problem except you will have to install longer riser pipes(usually) and a longer tail piece from the drain. Turn the water off, disconnect everything, remove the old vanity, set new vanity and measure length and diameter of pipes. Tail piece will probably be 1 1/4", risers vary at the shutoff end, take one of the old ones with you. Should be an easy DIY job.