I have a 50's ranch and we have 4" base and 2 1/4 ' window and door casings

Do you have a picture of the trim style you like?

Where do you plan on buying the material?

Casing is normally thicker than base.

I plan on having my contractor use stock 1x3 and 1x4 lumber. They sell it at Home Depot, for example. Attached is a photo of the style, but it shows it in 1x4, not 1x3.

Good choice on the trim style :)

A typical 1x4 is 3/4" x 3-1/2", and a 1x3 is 3/4" x 2-1/2".

In addition to various widths, you should have several thicknesses. The casing should be thicker than the base, and the stool will look better at least 1" thick.

Try to find a millwork supplier, or hardwood lumber supplier for your material. They can supply long lengths -- you want as few splices as possible, and can provide material any size.

You will want S4S (surfaced 4 sides) and ideally, sanded on one face. The sanding is important, especially if you are using a glossy finish. Otherwise, you may see knife marks in the wood from the sizing operation.

Consider popular as an alternative to pine.

I would ease the corners -- paint does not bond well to sharp corners, prime and sand before installation.