The two measurements in brackets are (half chest measurement // length measurement).
Half chest measurement: To figure this out, grab a t-shirt that you own and like the fit across the chest, then lay it flat on a table and measure armpit to armpit across the chest. That measurement is the half chest measurement.
Length measurement: To figure this measurement out, grab a t-shirt that you own and like the length of (as in, how far down your body it goes), lay it flat on a table with the back of the shirt facing up, then measure from the top (the back of the neck) down to the bottom of the shirt. This is the length measurement.