This chart may help you choosing the right size for you depending on wich shirt model (all sizes are approximate):
GILDAN (Heavy Cotton)
S (Small) Chest 46 cm / Length 71 cm
M (Medium) Chest 51 cm / Length 74 cm
L (Large) Chest 56 cm / Length 76 cm
XL (X-Large) Chest 61 cm / Length 79 cm
We recommend you check your own shirts and stuff, these brands are very common in bands merchandise so you will be 100% sure of what size you need.
Please contact us for more information.