Larry,
Dimple dies are not adjustable. A properly formed dimple comes from full application of "squeeze" to the dies. There is no way to under or over-squeeze to make a larger or smaller dimple, and still have an acceptably shaped dimple. Dimple dies (better ones, anyway) are design to go slightly past the ideal dimple shape, and the aluminum "springs back" into shape, hence the term "spring back dimple dies."
To make a larger size dimple, you would need a dimple die machined to produce such a dimple. Fortunately such a thing exists, and is known as a tank dimple die. Tank dies are designed to produce a dimple which is 7 or 8 thousandths deeper than standard dies, to make up for the thickness of the proseal under the rivet. I have heard of people using tank dies for underlying structure and standard dies for the skins, in order to achieve what you have described, but I don't think it's worth it.