The first thing we need you do is work out how long your chain is. The length of your chain is determined by counting the number of drive links in you chain (this is because chains stretch during their lifetime).
The diagram above shows the drive link, it is the tooth on the inside of the chain loop; it sits inside the groove of the guide bar. The easiest way to count them is to take the chain off the saw. Sometimes these drive links may have a code stamped into them, this will vary by chain manufacturer but it will identify the chain type for you.