Basically it's what's esthetically pleasing to the individual.
The point of the block is to highlight the area, make it stand out from the rest of the plane. I've worked with blocks that are 2" wider than the casing or BB, in general on the majority of doors and windows the block is anywhere from a 1/2" to an inch wider than the casing or BB.
Most Lumb yds carry prefab blocks with a few different rosette styles, you'll likely get a wider range from a Wood smith shop. You can have custom blocks made or you could make them yourself. As long as you have a TS or Miter box and a router.
I bought some nice CNC'd Clamshells from our local WS, cut and routed out my own blocks then glued the clamshells into them. My customer lurved the heck out of them as a dressing to the sill and aprons on her new winds.
If you're looking for historic reproductions then the price will jump, many profiles over 75 yrs old don't have existing cutters and may require sending an existing profile for a set of knives.
Work smart not hard!
Never bite the hand that looks dirty