I have some leftover exterior cedar siding that I want to use for molding in my great room. If I cut the laps I can have a 4 inch baseboard. I need to trim my front entry as well so was thinking either leaving it at 4 inches as well (very contemporary home) or cutting it down to 2.5 inches.
Is there a formula for the right size of floor baseboards to door casings?
umm there really is no "formula for door casings they are whatever you desire to be quite frank with you.
The standard big box store baseboards are 2 3/4" or some are 3 1/4" high and 3/8" thick at their widest point however this can deviate from manufacturer to manufacturer and profile to profile
The door casings are usually 2 1/4" wide and 9 1/16" thick allowing the door casings to run proud (or in other words stick out a little further than the base by 3 1/16ths". This is your standard colonial or ranch but again there are umpteen trims that are out there. you could have a wide stock you could have a square casing you could go with a craftsman style casing and you could go with a traditional casing. As far as the historical casings the possibilities there are endless. The trim that went into historical homes could vary widely. Some of the old shops used to have separate knives with the individual profile in any given home so as far as what is common the first set of dimensions is commonly what I see and may differ from geographical location to geographical location.
I hope that this helps and if it does not utilize Google and look at trim profiles such as :