Hi! even though we have not had a frost or freeze yet I think you might cut them back to 6 or 8 inches above ground. Leave them a couple of weeks for eyes to develope. One picture being worth a lot of words click on the Colorado Dahlia Society web page and go to digging and dividing dahlias. If you still have questions come back on this thread. You will find help here. Steve in Baltimore County.

An eye on a dahlia is a specialized set of cells that will go on to produce next year's plant/s. On a clump dug in fall, the eyes will look like small, raised white, yellow, and sometimes pinkish pimples . They are found on thickened toughened collar material around the stem of the plant from which tubers have grown, and are sometimes found on the underside of a clump. Some are latent eyes which haven't bulged out yet, so if in doubt, save that tuber and see what happens in Spring.

You must have an eye to produce plant growth for next year. The biggest mistake I've seen people make is hacking off the tuber right below the collar in the thinner neck. An eyeless tuber simply won't produce a plant- ever. "Blind tuber" is the term used to describe those. "Worthless" might be another. Here's one of those: tons of roots; nothing to grow. This tuber had no eye.
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To get the eye and some tissue behind it, one has to stab/cut into the collar around the eye and sever that whole eyed tuber from the clump. It gets easier with practice. Hacking into halves or quarter clumps isn't out of the question, but I don't do that. I find it's easier with dividing clumps in fall when they are full of moisture and softer to cut. They will dry and harden up over winter making cutting tougher, but the shoots will often be much more obvious in Spring.

Photos of eyes on the collar of a clump/tuber.
Here's an obvious eye with another formed at about ten o'clock above it. Raised lighter pimples.
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This cooperative variety has more obvious shoots, formerly eyes. There is an eye on the tiny tuber to the right, outlined in dark blue.
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An eye outlined in blue.
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central eye with brown, surrounded by 4 light 'pimples,' also eyes at 12, 3, 6 and 9 o'clock. An eye to the far upper right marked in blue on another tuber.

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Some tubers have a pink cast when dug, but really obvious eyes. Not all well-formed tubers have eyes. Get those that do first, then move down to less desirable weak/thin-necked tubers which are prone to breaking or rotting.
I just found a tuber from 2006 in my garage, in a plastic bag all year- still waiting for that eye to show up. Obviously, it had no eye, and no growth potential, but I was suprised at what good shape it was in for a year out of the ground and no special treatment.
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Eyes are much less obvious on these tubers, but it shows how one should be cutting into the collar material, not just cutting tubers off at the necks. As the tuber dries, the eyes will recess and re-emerge in spring like magic.
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Dig in and excavate those good eyed tubers from your clump! I don't know how people do it with knives; thin sharp shears are dangerous enough for me.
I will cut along the blue line to separate these two tubers, each with eyes/shoots.

Good luck with your dividing this year. It isn't hard at all once you get the hang of it.
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See, I told you a picture is worth a lot of words. Steve in Baltimore County.