NFL jerseys have become increasingly popular over the years in the retail market. Wearing these shirts is a great way for fans to feel connected to the sport they love due to the air of authenticity the jerseys lend. As far as official NFL jerseys go (replicas tend to follow a different sizing scale altogether), sizing can be problematic, as the jerseys are designed to take into account the fact that pads will be worn underneath. Since you presumably won't be donning pads, you have to consider a few key things in order to figure out which size of NFL jersey will fit you best.
Take your measurements. Chest, arm length and shoulders are all good to know. Also, be mindful of your current clothes and what tends to fit well. For example, do you require a larger fit than normal for your size because of broad shoulders? Many conversion charts in stores and online are written with the assumption that you know all of your measurements beyond the standard "small/medium/large" sizing guidelines.
Familiarise yourself with which brand of jersey you want to buy. The current official jersey maker for the NFL is Reebok, but limited edition and retro throwback jerseys can vary by brand. This is often the case if you are purchasing an older jersey online from a private seller. If the maker of the jersey only makes college jerseys now, try one of those on to get an idea of how it fits.
Use a conversion chart. These are pretty intuitive and easy to understand. You simply need to have the measurements you took earlier on hand, and convert from there. For example, if you are a male and your chest size is 42-44 inches, you would opt for a size Large.
Try a jersey on. Granted, this isn't always an option, especially if you are purchasing online. However, if at all possible, this is the best way to get an idea of what does and doesn't work for you. Even if the shop you are patronising doesn't sell NFL jerseys, try on an official college one. Also, keep in mind how you want to wear it. For example, are you going to be tucking it in? Are you going to be wearing a turtle neck or other long-sleeved shirt underneath?