Crowning achievements. Although there are some guidelines that can be used to size a crown (for example, make it about half the size of the base), there are many questions that come into play, like:• How tall is the ceiling?• Will the crown be convex or concave? • Will the crown step up and out from, say, a picture rail?Given these variables, sizing a crown isn't as straightforward as you might think. I like to purchase foot-long pieces of different sizes and profiles and construct mock-ups of the crown in the room. In fact, because most trim is readily available and inexpensive, this is a really good process to use for selecting all of your trim elements.