You can get anything but hopefully you will get a large dahlia. I started seeds from Gladiator AA-ID-R in 2007 and got a couple large ones but they didn't have enough petals and they were large open centered dahlias not worth keeping. The only good one I kept from these six seeds was a B sized variegated that was pink and purple.

Growing seedlings is like Christmas, you don't know what you have until it blooms and that is what makes it so interesting.

You could get any size & any color & no one can guarantee that it will look anything like the parent. Dahlias don't come true to the parent from seeds. Mostly, as huey said, it'll ba an open-centered throw-away.