You should also consider the size of the property and the distance the reader of the sign would be from the sign. A small property might look over done with a large sign and a small sign on a large property might look lost. Of course the further away the reader is from sign the more difficult it is to read a smaller sign.
The design of the sign is also important. A long line of text works better on a strong rectangle. A logo may work better on a square or round shape.
This is the most popular size for cottages: 14" x 23" . 7 x 23" is very popular for trailers but 14 x 23" is also used. 14" x 34" is my personal favourite. I like the strong rectangle with lots of space for text and graphics. But my own sign is 21"x23" because the design favoured a square.
The combo, a large sign with a smaller sign below, is useful if the information on the sign might change. Place the information that could change on the smaller less expensive sign.
Here is a picture that shows another option on how to mount a sign to a vertical post. Screw a plate to the end of the sign and then screw the plate to the vertical post.