The standard 954 chain is a 530 size which has a pitch of 5/8". You need an extra 6 inches of chain at the top and bottom of the chain run i.e. 12 inches extra total which equates to 19.2 links. You have to add an even number of links so you need an extra 20 links.
The standard chain length is 108 links, so if you are still running the standard sprocket sizes (which would be a bit unusual in conjunction with an extended swingarm) you would need a 128 link chain. If you aren't running the standard sprockets then you haven't given enough information.