Wow 18" would be one impressive "serving platter" dahlia! I've only been able to muster a mere 11" across here in a white ID with no name. Still pretty impressive for a flower.

On a side note, does anyone else find the Bing popup things the most annoying interruption in any online experience? Reminds me of telemarketer calls in days of yore.

We've had 15" Wyn's Conquistador get 15" without any problem-- & bigger when only 3-up (this is on late bloomers that are under the trees so don't mature very early- otherwisw we grow it as many up as it wants & it's still bigger than a basketball!) My brother-in-law grew a 17" one- the first dahlia he ever grew!
We have a seedling 06-01 that regularily gets upwards of 16" or so-
I was going to post pictures but now have to figure out how - Photobucket seems to have changed how you put pictures on here & I'm flummoxed! Or maybe it's the fault of Windows? Don't know but I'm stuck again!

I grew lots of dahlias last year and my Emory Paul was 15 inches across. Maki was second and was about 13 inches. I loved how big they got but since both of them bloomed late in the season, I only got about 2-3 blooms from them. The pic is of Maki but I can't find one of Emory Paul :-(