Ideally I would like to give mine 2' but in reality it usually gets down to about 16-18" by about 2' I turn out oddles of cut flowers from them. The big show dahlias definitely wouldn't like it, but for cut flowers it is just fine. And yes, do pinch them to increase the number of stems and feed them well.

Thanks for sharing your experience LizaLily. Most of my beds are 4 feet wide so I normally just have two rows of dahlias. I believe I'll try one bed with three rows spaced 16 inches on center and pinched hard.

Lizalily,
What do you feed them? I am in CO. and have clay soil. Put in grow beds with good soil but wonder what to fertilize with here in Denver. Last summer my garden was in MA.