Measure the balloon diameter.
If you know the largest diameter the balloon will get to before busting, you can use this to trigger the release. A length of string fixed at the top of the balloon down one side to a pull release switch fixed at the bottom would do it. As the balloon expands it gets to a point when the tension pulling on the string around the balloon pulls it taught. You could use a grenade pull pin type setup at the bottom as a release mechanism. This way you get to the highest altitude that the balloon can get to before releasing rather than a fixed altitude.