This message appears if the size entered in the Size Box on the Letter window of the Control Panel, or in the Letter Properties dialog is below 3mm or above 200mm, or is not a number. The Size setting refers to the height in millimeters of an uppercase 'A' in the selected font. If you wish to increase lettering above this you could drag one of the corner boxes or alter the Height or Width proportions.
This message appears if the size entered in the Line Spacing box in the Letter Letter Options or Letter Properties dialog is below 1mm or above 5mm, or is not a number.