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.

I use a 16 gauge Porter Cable nailer that shoots from 1" to 2&1/2" nails for trim. I also have a 18 gauge PC nailer and a 7/32" narrow crown stapler I use for trim, depending on the application. I use the stapler for door trim---the inside to the casing---that is the same size staples manufacturers use there. You can try pot luck and look on the Harbor Freight website---or in one of their stores and get a Chinese made copy of the real nailers for as little as $10----sometimes they work---and sometimes they dont. PC and other brands have kit deals---a 16 and 18 gauge nailer with a compressor/hose/accessories package---some for as little as $250. You may have to do some research to find the best deal. Google is your friend. :)

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.

Again to be clear, I was the one who made the window and color scheme selections early-on as I had to choose a whole-house scheme in order to settle on frame/sash colors so that we could order windows. The "third" architect was/is an assistant to the one who designed our house and was my primary consultant in finalizing the siding and trim colors (and hence the original dilemma and possible rescue of the color scheme). I had no expectations of this house looking historic and would have considered a structure of this size and simple architecture to be boring and monolithic were it all the same color. Perhaps I should have asked for help at an earlier point than I did but I do not transfer "blame" (if the word even applies) to anyone else for however the color scheme turns out. If nothing else I can always return to my original plan of having different trim colors on the upper and lower levels, which I didn't think looked that bad. Here is a screenshot I was able to save of the front elevation with all-green trim: