That depends. A 15-inch display is probably not a big enough monitor for digital imaging, but only a serious professional photographer needs to spend $1,000 on a 30-inch monitor. A related spec is the aspect ratio. With so much movie viewing on computer monitors, widescreen aspect ratios are becoming more popular. But you'll actually sacrifice overall screen area with a 19-inch widescreen instead of a 19-inch monitor with standard proportions.
Resolution is the other question. It's hard to get into too much detail without listing specs for every monitor size, but generally you want the highest resolution you can afford for your screen size. It's the two big numbers (i.e., 1300X1020) that add up to the single most important consideration.