Sizing when you reorder the same listing
Same listing, same item ID, same chart: the size you wrote down is still the size to send. A platform closing does not rewrite a measurement a seller printed. Copy the size text exactly as the row holds it, including any letter, number or suffix attached to it.
Sellers edit charts quietly. A listing that still answers can carry a measurement table that changed since you last looked. Compare the chart in front of you with the numbers you saved, not with your memory of them, and send the size the current table supports.
Two tools here do that comparison and nothing more. The apparel size chart turns a measurement into a labelled size using a table you select; the size converter handles a single figure. Neither invents a standard, because no platform publishes one this site has verified.
Sizing when the listing is gone
A dead listing takes its chart with it. What survives is your row: the size text, the item ID, and whatever screenshot or note you kept. That is enough to repeat an order only if the same item is listed somewhere else, which the item ID is the usual way to test.
Treat the old label as a label, not a measurement. Two sellers can list the same trainers and print different tables, so an M copied from a vanished page has no fixed chest, waist or insole figure attached to it.
Work from your own body or foot measurement against the table on the new listing. Where no table exists and you cannot measure yourself, the honest answer is to skip the item rather than guess a size and pay to find out.
Sizing across marketplaces
Labels travel badly between marketplaces. A size M on one listing and a size M on another can describe different measurements, because the chart is written by whoever listed the goods and not by a shared authority that everyone answers to.
Rebuild the wrapped address first, then read the destination listing and its own table from scratch. Numbers you derived for one chart do not transfer to another, and a conversion carried across out of habit is where sizing errors come from.
Where two listings describe the same item, measurements decide and letters do not. Line up the figures that matter for the garment, such as chest, waist, length or insole, against your own, and choose the listing whose table you can actually read.
What stays the same throughout
Your own measurements do not move when a platform does. Write them once, in one unit, and keep the note somewhere you will find it again: those figures serve every chart you meet afterwards, whichever marketplace wrote it and whenever.
The item ID is the other fixed point. Same identifier, same goods, whether the address in front of you is wrapped or plain. A rebuild changes the route to the listing, not the thing being sold, and not the body it has to fit.
What changes is the chart you are reading and the date you read it. Record both beside the size, and a reorder months from now starts from a dated fact instead of a faded memory of a letter. Two lines in a row cost nothing and settle most sizing arguments before they start.