Your coat should keep you warm during cold weather, but it won't if the sleeves are too short and expose your wrists. In addition, your coat looks far better when the sleeves hit at just the right spot, rather than extending too far down your hand because they are too long for your arms. To determine the correct length of the sleeves you need for your coat, you should have your arms measured by a seamstress or even just a friend.
Stand straight with your feet shoulder-width apart. Place your arms straight down to your sides.
Bend one arm so that your hand rests on your hip; your arm should be at a 90-degree angle.
Have a seamstress or friend hold a measuring tape at the nape of your neck and extend the tape across your shoulder, down your arm to your elbow. Write down the length in inches from your neck to your elbow.
Move the tape to your elbow, and measure the length to your wrist, writing down the measurement in inches when you are finished.
Combine the two numbers for a total that is the correct length in inches for your coat sleeves.