All of our main garments (the ones in the category pages) feature size guides like the one next to this text.
This includes the standard sizing, which could be in Small, Medium Large etc, and below, a unit of measurement, such as to fit chest size or equivalent ladies size.
To Fit means that this size is to fit that chest measurement. It is not the actual chest measurement of the garment, which would need to be slightly bigger to fit that chest.
To measure a chest, get a tape measure and pull it round the chest of the person to be measured, snugly but not too tight. See the graphic here for the locations of various measurements.