Looking for guidance from some of you finish carpenters.

I am building an apartment in the hayloft of a former dairy barn. I plan to use 4" square trim everywhere. We want a finished look, but one that is appropriate in a barn, if that makes sense. I plan to use butt joint casings at the windows and at a cased opening between the living and dining rooms, like the one shown at the bottom left of page 92 here: http://books.google.com/books?id=H9fuCs8SDcAC&pg=PA92&lpg=PA92&dq=butt+joint+window+casing&source=bl&ots=lXPvpebI_h&sig=bhEb4kjgEyLasG7pTCFszMqvCkY&hl=en&ei=VijWSZPhEcaY-gb3lKnKBQ&sa=X&oi=book_result&ct=result&resnum=2

I am having trouble figuring out what materials to use. In particular:

What size should the apron be? One carpenter suggested the same 1x4 stock as in the rest of the window casing. Another said that the width of the apron plus the thickness of the stool should equal the rest of the window casing.

Do I use regular 1x4 stock or 1x4 casing material?