Regarding the size of your signage, you should consider the dimensions of the installation or the mount and, secondly, the audience who will view it. For example, if you plan on putting your sign on a door, the signage should fit about one-third of the door. If it is bigger than a door, the sign could be larger too.  Your signage should cater to the type of audience who will view it. If you plan on putting up a restaurant advertisement for passers-by or pedestrians, you should not put too much text on it. The message of your sign should be viewable from afar.