This is pretty much what it says on the tin; currently if one attempts to use Locate Biome Center with too small of an area, an error message (currently in line 798 of conditiondialog.cpp) is raised saying the area is too small--but it doesn't say what the minimum allowable area actually is, leaving the user to simply keep guessing until they happen upon one that works. Given the lower bound is invoked just two lines above (cond.count * cond.biomeSize) I would imagine it would be fairly trivial to add "(Area must be at least [bound])" to the textbox message, but given I have never used Qt before I don't know the syntax that would be needed to be able to create a pull request for that myself.