It's not just length they measure, but the slope of the staircase too. The rails are produced in metre-long sections with a top tube & a bottom one, the two being connected by welded-in cross-pieces, and the rack is welded to the lower one. The sections are also welded to the stanchions that stand on the steps.
When ours was measured the bloke took umpteen photos with number cards and a ruler on each step for calibration back at the factory. Stannah would probably do that and produce an estimate for you. If it's a dead straight staircase it shouldn't be too blenchworthy, but once they throw in a curve or two, as with ours, it gets blood-curdling. We ended up forking out 12,000 €, about 7k of which was the track, and when the Inlaw Paw died after using it just once they told us the track was scrap but they'd give us 1500 € for the chair. We kept it.
BTW, if you do get it, make sure the batteries are still OK. Ours takes two, quite hefty items. The installer told us that if the chair stands unused for any length of time with the charger connected and they go belly-up, Stannah will charge around 100 € each, but you can get equivalents for around 50 €.