.... I want my sims to marry the townies but they don't have the body size I like. ....
Using SimPe you can extract any townie, change them in game to use the mesh you want (don't play them; just create the family), then use SimPe again to replace them. Then delete the family.
Note, sims seem to inherit from grandparents if they have any. You may want to change them as well.
I have done it this way before, there may be a better way.