The problem is that the error message is misleading, it is not too big, it is simply encroaching the roof boundaries. The best way to insert rooflights is to measure the X & Y coordinates from the dialog diagram below, e.g. entering 1,1 would place the rooflight over the edge of the roof. Use Cymap measure command to measure the X & Y dimensions from the virtual 0,0 datum. For the Width & Height, enter 0.1 & 0.1 so that you have a good chance of avoiding the roof boundaries. These can then be increased in size depending on how they are displayed in the Diagram.