The maximum size? Depends... how far away is your target audience? If you put a billboard up on say... Mt Lemon and want to reach say... 300,000? It would have to be really really BIG! (grin)

To your point... the size of any advertisement is directly related to its intent, design, and reach. Wanna get some attention? Create a billboard that spans two high on I-10 A pot of coffee on the top and a cup of coffee on the bottom with the message to fill up before you try to get to Elpaso. (grin)

(I grew up in Tucson, Sahuaro High School... Pima College... UofA and Davis Monthan.)

Good luck!!

Thank you for your comment, are you in Advertising?
We try to focus in the lenght of time people will have to see in order to build a certain billboard.

In advertising? Yeah, you could say that. www.soky.biz ... that's me.

Great, my brother used to live in Kentucky, well I am in outdoor advertising in Mexico, ass you may have seen in my web page rfandd.com, which is not so great yet, 'cause I did it my self.

I think a criteeon for size would be its placement. A very large roadside sign on the Great Plains might not be such a bright idea!