It like a taylor made suit. If the the tayor/builder is good it will fit and ride like a dream. Some suggestions:
-Try to deal directly with the builder, some customs go through a local shop, I like to to actually talk to the person building bike.
- Don't be afraid to ask about the builder's qualification and experience.
- Ask to see some pictures of other bike they have built and perhaps even contact other owners.
- Ask questions on forums such as this.
After you have done the above you will be confident in having your new custom bike fit. And if it doesn't a good custom builder will make it right.