Conversation

Small mug are best for coffee.

https://www.epicurious.com/expert-advice/small-coffee-mugs-are-better-article

Indeed, this has been my experience. I have a gigantic Starbucks mug at home that I rarely use because my coffee is lukewarm, at best, long before I finish it. The smallish size of classic diner mugs, or vintage Pyrex coffee cups are just about perfect.

Equally important, I think, is the circumference of the opening. The wider the mouth of a cup, the quicker it'll cool off.