I contacted Sue at Airlift to ask what part number might best fit the rear of a '96 Geo Metro.
When I measured the rear springs the other day, it looked like they were 4 inches across the inside of the spring, and the spring was about 7.25/7.5 inches tall.
Sue said that their item #60774 is a kit of 4" diameter by 7 inch tall airbags. (They recommend too short over too tall)
I looked up that part and saw that it's listed for a Nissan Murano. Seems like of wierd that springs from a Metro and Murano would be the same! I assume that the Murano springs are the same size but much stiffer?
Here's a link to that particular product:
Nissan Murano Air Bag Helper Spring Kit 2003-2007 - "Air Lift 1000" Suspension by Air Lift