OK, it is a little confusing because there are two aspects to this at first. On the one hand, you want to TEST that this solution is going to work for you. So you want an insert small enough to insert fit inside your cushion cover so that you can see the results for the thickness you have chosen. One cushion insert is also less of a hassle to return if need be.
Once you’ve found what works… you will know more of what needs to be done going forward to solve your sagging situation. Most of the time “small enough to fit inside the cover” will do it, because the area of the cushion foam that gets weakened the most is the area you actually sit on. Try one first and test your results. So many think it is just a matter of counting how many cushions they have and that’s not it at all… it’s about finding a solution that works for you and your situation. Then deciding to proceed with this DIY project or not. You also need at least 2″ of remaining foam height when sitting for this to work.
If you have any product usage questions once you buy, you can call me at the phone number on the product label. I enjoy talking with people to find a solution to help delay buying new furniture.