Most people use four rear wheels all around. If you stay with factory wheels, they are usually 5.5x15. Then they use four of whatever size the rear factory tires were. I have a Brabus and they came with larger tires/wheels. There should be many threads about people changing to the same size all around. If you make the change, not only will the car handle better, but you can then rotate your tires front to back.
Been very happy with my RT43's on my smart, handles the smaller bumps on the road much better, even the speed bumps in parking lots don't feel like I am going through the roof. I have aftermarket rims 6x15's all 4 corners
I always loved the looks of these rims, and was sad when they discontinued them. I was lucky enough to get a set when I got my car. I just hope nothing happens as they would be difficult to replace. My stock rims are pretty bent up for some reason.
I was just as happy with my Pure and the black painted wheel covers
There are wheel repair places that make them new again. The only thing is they may be far from you, so shipping cost would need to be added. (Maybe on a road trip you could bump into one and have it repaired, ie. for a meet-up).
Then I still have rainy day funds for the 15x6 size, and I have 15x7s for trade on top of that.
(I'll have to definitely do a better job on disclosing on them than the one who sold them to me though. I have three new ones and 4 that I bought from a forum member)...