A-frame swing set max length of 4x4 beams
There are two primary points of wood failure in this design: the beam span and post lift. Since the posts are primarily in either tension or compression, they're unlikely to be a failure concern. 

However, as they get longer, past say 12', they'll start to impose more diagonal stress on the brackets you've shown than they're intended to tolerate. That's my primary concern. If they fail laterally by weld tears, you're now relying on whatever penetration into the ground you've implemented to hold the thing up. 

If you go more than 12 feet above the ground I'd look for a more robust design.