This depends upon the style of the house and the style you are using for decorating. If you go too big on the trim, it will have the effect of lowering the ceiling. Having a high baseboard trim can do the same thing by visually shortening the walls.

that sounds a little heavy for only 8'. i'd cut the baseboard first.

What if I do 6 1/4" crown with a 5 1/2" baseboard, will that look nice in an 8 foot ceiling height?

Thank you so much. That settles it then :)

My ceilings are about 8' high on my 1940 classic Key West Colonial house. Doing a major down to studs renovation. I'm just ordering molding and trim now after consulting with designer, local spec developer and millwork shop. I'm going with 5 1/4" baseboard, 4 1/4" crown molding and 3 1/2" finger joint casings. These are bigger than what is on there now, but will give it a nice solid feel. Paint all walls, ceiling and trim in same color to raise ceiling height visually.