FWIW, Article on Craftsman style looks like good information. Depending on where you are located, you should try to find a local mill. Size of material should be related to the size of the openings, whether window or door. 1x4 is the "nominal" size, it is actually only 3/4"X3 1/2", generally not "heavy" enough in appearance for a window of any size. Lumber yard pine, even #1, select, or "clear" is typically knotty as h*ll, twisted, cupped, generally a PITA to work with and for all your effort will produce a less than desirable result.
Here in the northeast most areas have a mill within a reasonable distance where you can buy truly clear & knot free, easy to work, poplar lumber, sized to your specs, for about the same or a bit less than the readily available homeowner crap.
Assuming that you are installing average sized double hung windows (32"X60" +/-), and given the context (barn) you will want to use a casing that is minimally a true 4" wide; or perhaps 4 1/2" wide. My thoughts would run to a 5/4 (full one inch thick) by (actual) 4 1/2" wide. Boost the door casings to 5" wide.

Best of luck,
Jim

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