ours are about a healthy 1/4", and about a 1/2" deeper. but we have 2 different sizes...larger ones in the window corners and smaller as moldding filler...we decided to use them and the corner blocks rather than making all the miter cuts.

The dimensions for the casings with 3 3/4" rosettes are correct. You want the rosettes larger than the casing. Rosettes usually overhang the casing by about 3/16" to 1/4" on each side. Be sure to place the rosette right in the corner of the top and vertical jamb. You don't leave any reveal or else the reveal created by the vertical casing won't be correct. I'm real good on design elements because I can see the whole finished look in my mind's eye. And I'm wondering if the room won't feel a tad bit overpowered by so much large molding. If I was doing it, I might be tempted to skip the crown molding, or chose a smaller crown----maybe a 2"????? Just an idea.

In our last house we had windows/doors with flat casings and a simple backband and we plan to do the same thing in our new build. That house was 8' ceilings and I believe the flat stock was 4" + the backband but I'll ask my DH - he'll know since he made it. It was painted white and to my eye straddled the line between a traditional and a more clean lined look. In the new house it will be scaled for 9' ceilings and a vaulted LR but I don't know what those dimensions will be yet. If you're interested I'll see if I can find a picture where the casing shows well. Are you planning stained or painted trim?

When I moved into my current place, I had the same problem set. I actually did choose to replace all of my base molding throughout (yes, even in the closets), but kept all my original door casing. I have a variety of ceiling heights throughout the space and, while a crown treatment wasn't an option given my rooflines, I wanted to make sure that my base treatment created harmony throughout all the spaces and flowed without distraction. It gives the same level of finish and polish everywhere, which is a uniformity I wanted. I just made sure that it would coordinate well with my existing door casing (and my windows also have just an under-sill treatment like yours - no casing around the sides or across the top). It was one of the first projects I did, and I've really loved how it turned out. I did the 5.25" flat-sided ogee-top profile and it's worked out great as my style has filled in other parts of the plan around it.