Well, I'm sure that the anchor locker on our Islander is a good bit smaller than yours.
We had 300-ft of 1/2" 3-strand rode, and 30-ft of chain, to our horizontal windlass.
We now have 600-ft of 1/2" 8-plait line, and 30-ft of chain, and it feeds in and out better.
So, I would say that you could fit the same - alt least.
However, if you have a vertical windlass you need to make sure there is adequate "fall" below the windlass. Vertical windlasses need more "fall" than horizontal windlasses, due to the direction change in the line coming off the windlass.
So, I would strongly recommend 8-plait for your rode.
I got mine here:
http://www.secosouth.com/mm5/merchant.mvc?Screen=CTGY&Category_Code=8_plait_nylon
Brian