While some rules are made to be broken, dress codes are most definitely not. So when the occasion is special, the clobber should follow suit (morning or tux, preferably). Browse formal tailoring, shirts, shoes and even accessories so you don’t end up being the wally in the jeans.
Raise your arm and bend it slightly at the elbow (like you're looking at your watch). Using a tape measure, start at the centre of your back, just below your collar. Measure from here to the end of your shoulder and note down the measurement. Next, measure from the peak of your shoulder to your elbow, and finally, measure from your elbow to your wrist (where you want your cuff to sit). Add all three measurements together to calculate your sleeve length.