Building my own to clear the anchor and other stored items. I can make than most any size other than the depth. Looking on the web I see that the volume varies anywhere from 520 cubic inches to 864 cubic inches. The speaker will have about 50 watts continuous power, non vented. What is the rule of thumb and why would I go to a larger or smaller cubic inch?

What brand is the driver? Some of the speaker manufacturer websites will have data for the ideal volume for each driver they make on their website. The volume can vary a little from brand to brand and driver to driver and whether the box is ported or sealed. After that I think you just split the difference and go after the middle number.