Keystones undoubtedly add some character to your window trim, but some care does need to be taken when selecting which one to have installed. A keystone that is too small won’t fit properly into the trim, leaving a portion of it exposed. A keystone that is too large (as seen below) on the other hand, can be disproportionately large and also look out of place.
A keystone that is obviously too large for the trim that "contains" it.
The key question (pun fully intended) then is, how do you select the right size? As a rule of thumb, keystones should be 2-3″ larger than the trim they’re being fit into. This becomes difficult with smaller window trims that are only 4″ in size like Trim 001, and in such cases I have seen contractors actually cut the bottom 3″ off a keystone so that it is height proportionate. Don’t forget to re-apply the cementitious coating after doing this.