I tried a dahlia in a huge (7 gallon) pot this year. It's a mid sized Dahlia, thought I can't remember the name. I'm getting lots of foliage (although it seems small to me?), but no buds yet.
I've carefully watered and amended the soil with compost manure when I planted. The pot has excellent drainage. We have had a rather damp season, but I do move it under the covered portion of the patio if I think it's getting too much rain. Then right back out into full sun. Typically gets 8+ hours per day.
I did plant it sideways as I was unsure which way was up (read that advice here). Gladiolus in the neighboring pot are doing wonderfully under the same condition- just budding this week.