How to Measure your Sleeve Length
Take one of your best fitting shirts and lay it flat with the back facing up. Locate the centre of the base of the collar, from this point measure across the top of the shoulder and continue along the sleeve to the end of the cuff.
Alternatively you can ask someone to measure from the centre of your neck, continuing across your shoulders and down your arm to just above the palm or to your desired length of sleeve.
If the measurement falls between two sizes, choose the longer size.