18 x 24 inches is a great starting point.
You can really go any size here, but you definitely want your sign big enough to be the star of the party.
Many hosts display the sign on either a table (next to the gifts and cake) or lean it against a wall.
I recommend making your sign taller (portrait size) for table displays, and wider (landscape size) for walls.
TIP: If you go the table route, use a small easel to hold it up.