One of the most important questions that we all have about trim is, What size should it be? Getting the size and proportions of trim correct can make a huge difference in how a room is used and enjoyed. If the trim is too small or too large, it can be like a clown wearing a minuscule hat or supersize pair of shoes.
While there are some guidelines, they are, in fact, just guidelines. No rule should replace what you like and what you feel is right. Start out selecting your trim by first defining the story you and the room want to tell. From there, purchase some samples and have mock-ups built. This is all a small price to pay for getting your trim right.
Here are some guidelines from Houzz.com and Bud Dietrich, AIA — for sizing the trim in a room.