The length of roll is defined as the number of meters of material, which is used to print the necessary data to the receipt etc. The length depends on the substance of used material and defined diameter of roll.
The diameter of cash register rolls is determined by the type of device in which the roll is used. The aim is to have the utmost meters in a defined diameter and thus save time and material.
Also the width of roll depends on the device in which the roll is used.