Numeric Sizes and S–XXXL: Avoid Guesswork
Use the size chart for the exact garment instead of converting between numeric and letter labels by assumption.
Treat labels as identifiers
A numeric size can represent a different system from a letter label, and conventions can differ across garments. A familiar number does not automatically establish chest, length or shoulder measurements. Check the chart or ask what the label means for that item.
Compare the dimensions that matter
Look at more than one measurement. A garment may fit at the chest but differ in shoulder or length. Send the product link and your measurement question to get a relevant answer instead of asking for a universal size conversion.