Haven't had experience with a larger endowed man so this is going strictly off of experience with toys. I'd say 8L x 1.5W would be ideal for me. Well...possibly 9 since 8 doesn't bottom out in me but I don't have a toy I like that long.
My favorite toy is an 8 inch pink wand but because it has a hard curve at the end it can't bottom out. Once I finally learned how to measure myself correctly I realized I have quite a bit more depth than the average woman. And of course I've never been with a guy who could reach the back.