What size trowel for 16x16 tiles

X10012

I'm sure all of you seasoned DIY'ers and prof out there should be able to make this a quick one.

I just redguarded my hardie, and I'm ready to start tiling my shower surround. I plan on putting up 16x16's, but I need a clarification on what size trowel to use.. FYI I do have a 1/8 dip on the back wall near the ceiling. I'll have to take care of it with some extra thinset (thought I'd add that just in case it changes some of your answers).

So the question is, what size trowel for these 16x16's (porcelain). I was thinking 3/8, back butter everyone and checking for the standard 90% coverage... Sound about right? Is 1/2 a little to big?

X10012

Just read the back of my Flex bond bag which mentions 1/2 for anything over 12x12... Is that suggested by the pros? Also, do I need anything other then the flex bond thinset, like a special adhesive because they are large format?