I don't know how you ladies do eternity rings. I'm thinking of getting one for my wedding band but the thought of not being able to resize it stresses me out even though I love eternity bands!
Usually you are advised to "go bigger" with eternity rings and get sizing balls if needed. But, you can always get a "near eternity" with room for sizing.
Wearing multiple rings though also "tightens" things up. If you wear an eternity ring in between say a wedding band and an ering, it being bigger is not necessarily an issue.