There's no "limit" from a coding stand point (except maybe what the python engine or the N900's hardware or Maemo five can support). It's just that if you make it too big, it will keep widening the status icon area, hiding more and more of the rest of the status bar. I don't know if it maxes out towards the very end, but by then it's already a usability PITA if the user using the clock style wants to open the menu of an app that has its menu in the status bar.