According to the description of this NASA image [1], they flew a 7-million-cubic-feet super-pressure balloon in 2009. Approximating as a perfect sphere, that's

According to the description of this NASA image [1], they flew a 7-million-cubic-feet super-pressure balloon in 2009. Approximating as a perfect sphere, that's around 120 feet in radius / 80 yards in diameter. And "When development ends, NASA will have a 22 million-cubic-foot balloon", which is around 170 feet in radius / 110 yards in diameter. These things also look squashed when inflated [2], so they're probably even wider horizontally. Basically, you can fit an entire football field inside along with some stands. 1. https://commons.wikimedia.org/wiki/File:NASA-NSF_super_pressure_balloon.jpg https://commons.wikimedia.org/wiki/File:NASA-NSF_super_press... 2. https://commons.wikimedia.org/wiki/File:Super_v_zero_pressure_balloon.jpg# https://commons.wikimedia.org/wiki/File:Super_v_zero_pressur...