I came across this pattern for crocheting flowers, and I finished one like in the middle of this picture.
http://myoriginalpatterns.blogspot.com/2005/12/appliques-for-baby-items.html
However, I would like to make another that is slightly enlarged (about 2-3 mm out on every side), like concentric circles - only with flowers - so they can be stacked.
Could anyone offer me a tip on how to do this?

If you go up or down hook sizes you flowers will get bigger or smaller. You are not worried about gauge but just size. When your work isn't up to gauge you go up or down a hook size or two. This will give you different size flowers.

thanks, I will try that!

Try adding two more rounds to the flower.

On the first extra round, make chain loops behind the last row of shells (petals) -- try a chain of 5. Slip stitch around the base of first dc of the previous round. Chain 5. Slip stitch around the base last dc of the petal in the previous round.

On the second extra round, ch 2, dc 5, ch 2 into each chain loop you made.

You now have a second row of petals on your flower.