Length of winter rods
The length of a winter rod is measured along the tip, not along the whole tackle, and it has nothing to do with range: there's no cast here. It decides the sweep of the hand over the hole.
What a shorter tip changes
The hand ends up almost right over the hole, and the movement comes from the wrist. That's exactly what's needed when the lure has to be kept in a constant small oscillation: a short lever doesn't dampen the frequency and doesn't tire the hand the way a long one does.
What a longer tip changes
Travel appears — as much vertical movement of the tip as it takes to flick the lure up with one motion of the wrist; with a short tip the movement hits its limit halfway through. That's why longer tips are used in spoon fishing. A longer tip also springs on the hookset and absorbs the jerk, which matters where the fish is large and little line is out under the ice: the line's stretch is a percentage of its length, and a short one leaves almost none of it.
Why length is tied to the spring bobber
Long tips more often sit on rods without a spring bobber: for a heavy lure and a sharp bite a separate blade isn't needed, and the tip itself takes over its role. Short tips are the opposite: there, the spring bobber is the main working element, and length is chosen so the hand can reach it without a wide sweep.
Where length hits a limit
There's no ceiling over the hole, but there's wind, snow and the angler himself. A long rod has nowhere to rest, it snags on clothing and catches the wind, and once you're sitting on a box you can't swing it anymore: the elbow hits the knee before the tip reaches the top of its arc. So the upper limit is set not by the fish but by the angler's posture: what's comfortable standing over a hole in open country becomes awkward in a tent or sitting on a box.
Handle length is a separate question
It's measured separately from the tip, and by different reasoning altogether. A short handle gives the wrist freedom, a long one lets the forearm brace against it and hold the tackle for hours without tiring. So two rods with the same tip can work quite differently, and handle material matters here just as much as its size.
Why we don't name an optimum
Because there isn't one in the tackle itself. The method of fishing sets the length, not the rod: continuous hand-worked action calls for one length, flicking calls for another, and there's no middle ground between them that works everywhere. Add to that the angler's height and whether he sits or stands.
What we don't write here
Numbers in centimetres. The figures that circulate online as the "optimal length" are somebody's habit, not a measurement, and these are exactly the kind of numbers that get copied from one source to another without checking. Shops also split their range by length, but that's a division of merchandise, not a property of the tackle: the very same rod ends up in different rows in two different catalogues.