18H x 24W

all pretty standard here 24x18

Where most of my activity occurs we're limited to 18"W x 12"H plus other specific guidelines. 

Richard Robibero, ABR, SRS We use an 18 x 24 sign panel

Local code restricts us to 24x24. However, I quit using signs and my clients actually prefer that.

Here in Tucson they are all about the same size,   I would say 24 inches by 24 inches, which leads me to belive they need to be a certain size.  I have seen banners at events, I have one of my own I break out sometimes.  These are for residential, commerical signs are much bigger.

we use a white post with a 36" sign....and riders too...

Commercial 4 X 8.

Residential 36x24

Limited service 24 x 20

For condos I use telepathy.

There is no such thing as a sign that is TOO LARGE....only non-comforming.

18 x 24 is good. Most sign riders are 24" long, and 18 x 24 is enoght space to include all the broker and contact info. You can always add riders top and bottom depending on your local regs.

I have many normal oblong signs, and a few arm signs. 

30 X 24

Richard Robibero, ABR, SRS - I have seen 18"x24" lawn signs.

The horizontal one (24"x18") looks too big compared to vertical one - however, does not look that pretty.

I have 24W x 18H which my team uses in residential communities where its house next to house. I also have a few 36W x 24H for homes that have a bit more land. I find that some neighborhoods have restrictions to the 24x18 or smaller.

Most are 18"X24". There are a few communities that will only allow 12X18 for regular listings. Check with the towns in your area and go from there.

18x24