One rider worked out how electric bicycles are actually sized, then wrote down the measurements, the geometry numbers and the shop questions that turned out to matter. Free to read, nothing for sale.
A geometry chart is only useful next to a second set of numbers, and most riders arrive at the chart with nothing to compare it against, which is why the manufacturer's size recommendation, a single height range spanning five or six inches, ends up doing all the work. Five measurements taken at home change that arithmetic entirely. Each one answers a specific published figure, not a vague sense of fit, and each can be taken alone, in a hallway, with a hardcover book, a pencil and a tape measure.
Inseam, taken with a book against the wall
Stand barefoot with your back and heels to a bare wall, feet about six inches apart, and pull a hardcover book up between your legs firmly, spine level, the way a saddle would sit. Mark the top of the spine on the wall with a pencil, step away, and measure from the mark down to the floor. Do it three times and take the largest, since the first attempt is almost always too gentle. That number governs two published figures at once: standover height, which must sit below it with clearance to spare, and the saddle height range printed in the seatpost specification.
On an electric bike the standover margin matters more than it does on an unassisted bicycle, because the assembled weight is higher and a stop on a slope leaves you supporting a machine that will not politely fall away from you. Two inches of clearance on a step-over frame is workable for most riders; three is comfortable. Compare that against a step-through or mid-step frame, where standover stops being a constraint altogether and inseam becomes purely a saddle-height question, and you can see why the same rider often fits two frames that read very differently on paper.
Torso and arm length, which decide reach and stack
Measure your torso from the bony notch at the top of the breastbone down to the same book spine mark, which is easier alone than it sounds if you keep the tape flat against your body and read it in a mirror. For arm length, hang one arm relaxed and measure from the knob of bone at the outside of the shoulder to the wrist crease. Together those two numbers, not your height, are what reach and stack are describing. A long-torso rider on a short-reach frame ends up cramped and upright; a short-torso rider on a long-reach frame carries weight on the wrists.
Reach is the horizontal distance from the bottom bracket to the top center of the head tube, stack the vertical, and the pair of them is more honest than effective top tube length, which changes with seat tube angle and flatters some frames unfairly. If a manufacturer publishes only effective top tube, note the seat tube angle alongside it and treat the comparison between two bikes as approximate. Riders whose torso and arms both run long for their height should read the larger size's reach before they read its standover, since stem length can trim reach but nothing shortens a top tube.
Shoulder width and hand span, the two nobody takes
Shoulder width is measured across the back, from the outside of one shoulder joint to the other, and it maps onto handlebar width, a figure most electric bike listings do publish in millimeters. Bars appreciably wider than your shoulders open the chest and slow the steering; narrower bars quicken it and crowd the hands. Hand span, measured from the crease of the palm to the tip of the middle finger, maps onto brake lever reach adjustment and grip diameter, and it explains why a rider with small hands can find a hydraulic lever unreachable from the hoods on one bike and perfectly placed on another with the reach screw wound in.
The Consumer Product Safety Commission is responsible for the safety requirements bicycles sold in the United States are built to, braking performance among them, and lever reach is where that performance meets an individual hand. A careful reader checks whether the levers on a given model are reach-adjustable at all before checking anything else about the cockpit, because a fixed lever is one of the few fit problems that no stem, saddle or seatpost swap will solve.
Writing the five numbers down
Keep the five on a card in your wallet or a note on your phone, in inches and millimeters both, since American listings mix the two freely. When you open a geometry chart, read standover against inseam first, reach and stack against torso and arms second, and handlebar width against shoulders third, and you will have narrowed a lineup of four sizes to one or two before you have looked at a single photograph. At the shop, the questions become concrete: what is the reach on this size, are the levers adjustable, how much seatpost is exposed at my saddle height.
The measurements take about fifteen minutes and stay valid for years. They convert a spec sheet from a page of numbers about a bicycle into a page of numbers about whether that bicycle fits you.