They're supposed to last ~100 days and are pretty large. The time was one of the big points they had to address to get the economics right. The balloons had to last much longer than a normal balloon would.

https://lh3.googleusercontent.com/-1EOC6WWfgA8/UbvU_yFNsKI/A...

They last about a month IIRC, and they're not particularly big - the equivalent of 15 or 20 party balloons, I would estimate.