You need to balance the size of your crown with all the other trim. It will also depend on the style of the trim itself.

If traditional/colonial style and your baseboards are 5 1/2" - 6" (or more), your crown could be 4 or 5". If that is the case, your door casings would need to be at least 3 1/2". But that can also depend on the thickness of your walls, and the door height. Are your doors 6' 8" or 8' high?

It all needs to work together.

Search for articles on the "Golden ratio".

I have 9' ceilings in my house and have a 5 1/4" wm45 moulding with 1" of beaded mullion wm-956 below to complete the entablature. You could decide on some candidate mouldings, buy 4' sections of the crown, tack it up and live with it for a few days to make a decision.

If your home is contemporary, then crown molding might not even be appropriate. Hard to start and stop the detail in a contemporary home. It's a detail for homes with vaulted ceilings.

The most popular question "what size crown should I use on my 8' ceiling? After running a few miles of different types of crown molding the general rule of thumb to keep everything within proportions is:

7-foot ceiling - 3" or smaller

8-foot ceiling - 3 1/2"- 4 1/2"

9-foot ceiling - 5 1/4"

10-foot ceiling - 6 1/2"-7 1/4"

The crown molding spring angle is also important when selecting a crown, as @Steve Grimes suggested you can get samples of crowns from every supplier and try them out. For every crown job, we put up a few pieces to see what works best, if a build-up is in question, we will make a prototype and put it up and see what combination looks best.

When dealing with 12' and up ceilings, it becomes a different story, most of the time you will be dealing with crown buildups, etc

I need to know the style of the house and the crown.

i'm late to the party. You've probably done it by now. I was reading in discussions where this lady wanted 5" molding. Her husband didn't think it would look good. Too overwhelming. I don't remember the ceiling height. However, they got the 5" and he even said after it was up, that she made the right choice.

With that, I decided to go for it. I have 8 ft ceilings. I couldn't find 5" but I got 4.5". I love it. I thought it might be too overwhelming for my small rooms until I saw it up on the wall. It's not ornate, just plain. I also got the same size baseboards and I like them too. Oh, and because of cost, I was going to get mdf; however, after research, I went with wood. I don't regret that at all. Family member has that plastic looking crown. Family member talked her into it. I could tell without getting up close. Ugh.

I say if you can do it, do it. I wish I had marked the lady's email so I could comment on it. She was right.