Your problem isn't the size of the stools. It's the size of the human body. You can put your guests on pogo sticks, but they will still have to either sit sideways or sit far away from the counter, due to the fact that they have legs.
You can cheat these somewhat, but the general guidelines are:
At 30-inch-high tables/counters, allow a 24-inch-wide by 18-inch-deep knee space for each seated diner
At 36-inch-high counters, allow a 24-inch-wide by 15-inch-deep knee space
At 42-inch-high counters, allow a 24-inch-wide by 12-inch-deep knee space
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