Motion of the ocean...blah blah blah... Come on, we are talking about dildos people. Not life partners. You can pick any size you like. Yes, bigger is better, up to a point. Where that point is will vary from person to person, and also day to day.
I had lots of fun exploring with huge hole stretching toys that were 3" thick in diameter. I may do that again some day, but not I like thrusting with toys that are 6.5" in girth up to 8" in girth.
-When it comes to penetration, thicker = more intense.