99% of Thermal receipt printers sold use 80mm (w) x 80mm (d)
Here are links to the box of 24 and 48 you can buy from HotPOS:
Box of 24 : https://hotposnew.au1.cdn-alpha.com/supplies/thermal-rolls-80x80mm-box-of-24.html
Box 50 : https://hotposnew.au1.cdn-alpha.com/supplies/thermal-rolls-80x80mm-box-48.html
Taking the measurements
Firstly you need to measure the width of your till roll. Do measure it precisely to be sure it will fit into your equipment. Refer to the user guide that came with the equipment if your are in any doubt.
Secondly, measure the full diameter of the paper across the middle of a new till roll (include the core in the centre as well). It is not as important being as precise in measuring the diameter of your till roll with measuring the width. You may indeed use a smaller size – it simply will just run out quicker!
Understanding the types
There are two types of till roll; thermal and single and multi-ply:
Thermal paper
This is heat sensitive paper. These are used in machines with a thermal print head. A quick way to tell if you have thermal paper is to run your fingernail over the paper. If it leaves a black mark you have thermal paper. If it just leave an indent but no black mark you have plain or bonded paper (refer below)
Single-ply and Multi ply paper
These till rolls are used in impact (dot matrix) printers. You can tell if the equipment you are using is an impact printer by seeing if it uses a ribbon. You may then use a single or multi-ply till roll.