Tammy,
If I may be so bold, as a once upon a time professional hat maker (including top hats), I offer this advice. The picture shows a rough pattern of a top hat--not to scale. Two things you may not think of. The brim should dip down in the front and back, and go up on the sides. This is accomplished by two things. Notice the bottom of the crown pattern has wave to it. This is important to the shape of the brim. Second, if you use thin enough leather you can sew a spring steel wire into the edge of the brim, which will keep the brim in shape. I call it the potato chip effect. If you just sew the wire in it won't do anything special, but if you force it larger than it wants to be, it will bend like a potato chip, and that combined with the crown shape is the secret to having a great looking hat. This works also for Western style hats. The wire being too large for the brim forces it to bend up and down.
This pattern is for a stovepipe hat--straight up and down. If you want the top to be wider or narrower--both fashionable at one time--you simply give the overall crown pattern a curve, ie. a smile or a frown. Ask questions if this isn't clear.