I'm in the market for a puffy coat, but I've never been very good at layering. I'm always too hot or too cold. I'm rather new to winter activities, and I think I've settles on the Micro Puff. But I can't decide between a Med or Large. The Medium fit well over a base layer and a light fleece with room to move comfortably. But would I get more use out of going to the Large so I can fit it over more layers if need be?
I prolly won't be backpacking in anything under 15 degrees, but for general backpacking, skiing, snowshoeing.. is the Micro overkill? Should I go for the Nano? And what size would you recommend?

I don't have a Micro Puff, but I do have a similar, lighter jacket for warmer temps. I think it would probably work into the teens with the other clothing you mentioned. The only way to be sure is to test it yourself. Each person adapts to the cold differently.

Actively moving in the teens, you'll likely want a heavier fleece and some type of wind-blocking layer like a windshirt or soft shell jacket. The Micro Puff would only be needed for when not moving.

It sounds like a large would be too big, and that might result in drafts.

is the most you'll wear underneath it the base layer and light fleece? if so, then the medium is the choice. if you plan to add a down vest or 300 wt fleece then consider the large. my older version of that coat is very roomy in the torso area and allows for me to wear a down half sleeve shirt underneath it without any compression and that's how i use that jacket.

the roomier jacket absolutely won't be as warm stand alone as the perfect fit jacket, but it's what you're wearing underneath that will make the difference.

for what it's worth, for temps in the lows 30s, 20s and below i wouldn't mess around w/ the nano puff...