If you are replacing the door jamb or casing around the jamb, the very edge of the jamb should be exposed before the casing is secured to it. This is the “reveal”. The casing spans the gap between the door jamb and the wall and hides things like nails and shims so that all you see is the attractive wood work around the door.
I can’t say that there are any hard and fast rules on the sizes of reveals, but generally the size of the reveal is 3/16” of an inch. However, depending on the size and profile of your casing, the reveal can be larger, but I wouldn’t make it smaller. If you place the casing too close to the edge of the jamb, you can run into trouble with the hinges (this could happen if the profile is tall and the hinges are large in diameter). If you have a large door and have tall baseboard and casing, it would certainly be appropriate to extend the reveal. Play with the size until you are satisfied. The larger the material you are using, the larger you can make the reveal. By the way, it is much easier to paint the casing first and then install it. After that you can just fill the nail holes and touch up those spots. It’s easier than trying to cut around the entire perimeter.Replaced Shower Cartridge but It still Leaks » « Trimming Attic Ladder Stairs