I usually go by how big is the room, how high is the ceiling. usually small rooms with 8 foot ceilings I use the 3 5/8 or the 4 5/8
rooms with 9 or 10' ceilings such as a bedroom living room dining room etc. I use the 4 5/8 or the 5 5/8
and even get involved with 2,3 or 4 pieice built up crowns.
and this usually looks good.
go and buy several of the sizes, in 4' pieces and to use when selling jobs and show the people what they would like.
this way it gives the room a good balance.
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David's right, there are 'rules' for the sizes, based on the HEIGHT of the ceiling, as he said. Generally it's 3 1/2 (about a 2 1/2 in. rise) in an 8ft. room, etc, just as Get Your House Right suggests. The size of the crown is based on the same system of proportions as the chair rail. William Pain writes about some of those proportions in the same book that Brent quoted for the Chair Rail article: The Practical House Carpenter. I borrowed some of those rules--along with some from other pattern books of the same period-- for this short web-article: http://www.garymkatz.com/ChartsDrawings/proportion.html
So according to classical principles, the dimensions of moldings (base, chair rail, crown) are determined solely by ceiling height. The size of the room is not a factor. So I should use the same size moldings in a small bathroom as in a large living area given that the ceiling height is the same. Is this correct?
Best I can tell from the book, yes. I've had other books which have given "guide lines" not charts and specific numbers. Remember that it's proportion small trim in a tall room disappears, I just got an add on to a job by pointing out that the "baseboard" in the new house was door casing and that EVEN 4-1/4" MDF base would look better in the room (could not get the 5-1/2 as suggested, cause I did not have a sample). But now I'm redoing the whole house.
I applied the formula to a built in's crown and I was amazed at how much nicer my piece looked than what I had been doing in the past (proportion to size not height)