Bigger be better....... Up to a point.
Your desktop space is critical, especially if your monitor must be placed between shelves or storage units. Watch those external dimensions as some monitors have a pretty generous benzel that can add an inch or more to the width compared to other monitors.
And your seating position relative to the monitor can be an bigger issue. Do you need space infront of the monitor for your work papers? If so, your monitor will be further from your eyes and you could easily justify a larger monitor. Just remember that you never want a monitor that has a large amount of display space outside of your peripheral range (especially true for movies or games).
At the same time, your desk position may justify a smaller monitor. In my desk situation I have a corner desk with the monitor placed to the front edge of the desk. My writing station and reading stations are to the right and left. I have a slide out keyboard drawer, but even so, I am very, VERY close to the monitor... maybe 21-22" distance. A wide screen is possible, but anything bigger than a 24" widescreen would be too large for comfortable viewing of on-screen movement. Personally I can afford the 30" and would use the monitor for shared display purposes, but the 24" is as big as I can visually handle and may even drop to a 22".