This is a bigger picture of layout if that helps!

Our island is 9 feet. We put the trash at the end of the island down by the main sink this way it can be used for prepping at island and for scraping plates into before going into dishwasher. We left some space next to the prep sink for a landing area for things coming out of fridge and there is also room for someone to prep there as well. Prep sink is 19.75in x 20.5in. I would not go smaller than that.

Prep sink:

As far as the size of the prep sink— think carefully about what exactly you’ll be doing there. Washing veggies? Draining pasta? Washing dishes as you go? We have the Kohler stages 33” and have never thought it was too big. Maybe too small occasionally. But mostly just right.

The location of the ovens behind the patio door could be problematic…I’d prefer a range and a microwave in the island.

Your actual cabinet design eliminated the end counter overhang shown on the floor plan. You may want to move the prep one cabinet over to maintain some free counter space on the end.