None of the jerseys are measured by "chest size" like a suit jacket would be. The best way to find out what size jersey you would take is to find a t-shirt or sweatshirt that fits as you would like your jersey to fit. Lay it down flat on a table and measure across the chest just under the sleeves. The sizing chart for the senior jersey tells you what that measurement is on those jerseys.
For reference, the size large senior jersey measures approx. 23" across the chest. An X-Large jersey measures approx. 24.5" across the chest.
If you wanted a pro-style jersey, take the measurement across the chest and double that to give you the size jersey to order (ie. if you measured 23" across the chest, you would order a size 46 pro-style jersey).