If you want bigger crown, just make the circle bigger.
Ok, here's how you make it:
First, choose a stiff fabric for the brim and iron on vlisofix. Cut out 2 pieces (with seamallowance).
Cut out 1 crown (adding seam-allowance).
Take the brim-pieces, lay them RST (right side together), and sew along outer brimline. Trim seam, turn, fold in opening seamallowance and stitch close. (I recommend using a ladder-stitch.) Iron flat and topstitch along outer brim line.
Take crown-piece, fold and pin seamallowance to wrong side and stitch in place.
Gather the crown and check on you doll's head for best fit (the brim should go infront of the ears), pin to brim and stitch in place. Take a nice piece of ribbon and stitch that over the brim/crown seamline to cover it. I used the same ribbon to tie the bonnet under the chin. Voila! Your done!