8" minimum in my eyes and prefer Wet Sounds, depending on what kind of boarding you do, if you surf more I would run either the Icon8, or the REV8, or if you do more wakeboarding I would do the REV8's or the REV10's.
Surface area rules in an open field environment and based on surface area one 8-inch speaker is nearly like having two 6.5-inch speakers. Plus, you will undoubtedly have more pod displacement with an 8-inch speaker and that determines the low frequency extension. So an 8-inch will be warmer and play deeper.
When it comes to midbass extension, a higher quantity of a smaller speaker will never compete with fewer of a larger speaker. This also makes a 10-inch tower speaker advantagous over an 8-inch speaker.
Just a should out to David here, man you always have the best answers. Knowledgeable but not too technical that are easy to understand. You are an awesome member of this forum. Cheers.