These batteries are nominally 26mm in diameter and 65mm long (which is where their model number comes from...) but they are fattened up a bit by their outer wrapper and (usually) a protection circuit-board (PCB) mounted at one end, which turns the battery off if it is overcharged, over-discharged, short circuited or asked to supply too much current. (The PCB is re-set when it's put back in a charger). These batteries are the same length and voltage as the more common 18650 cells, just wider, and their bigger size allows them to hold more energy, which translates to longer run-times. Torches that take 26650 batteries can sometimes run on 18650s, if they are padded up a bit. Unfortunately 26650 batteries are far less common than their slimmer siblings, and a bit more expensive.
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