This is a guide to the sizes of Uniqlo's T-Shirts. Please keep in mind that Japanese sizes are smaller than North American sizes. You may want to get one size bigger than what you normally wear, unless you prefer it to be tight fitting.
The top set of measurements are for the shirts themselves. So one easy way to find the right size for you is to measure a few of your favourite fitting T-Shirts, and compare the measurements to these. Just lay them flat on a table, and measure across, as shown on the chart. Use centimetres.
The bottom set of measurements are more of a rough guide based on your actual body measurements. For instance, my waist is 92cm and my chest is 102cm. So according to this guide, I am a size XL (barely). This is indeed the case for me - I have tons of Uniqlo shirts - all are XL. It also depends on how you like your shirts to fit. If you are in the middle like me, decide whether you want a tighter fitting, or looser fitting shirt. I could stand to lose a few pounds though, and when that happens, I'll be an L ;)