The 7 percent solution. When sizing a baseboard in a traditional-style home, a good starting point is to use a ratio of 7 percent. So if your ceiling height is 8 feet high, try a baseboard that's about 7 inches tall.
For casings, 50 percent is good. Generally, vertical trim elements such as door and window casings should be smaller and have less heft than baseboards. So I've found that a good rule of thumb for sizing window and door casings is to keep them at about 50 percent of the height of the baseboard.
As always, this isn't a hard and fast rule. Casings that equal the dimensions of the baseboards certainly can work quite well.