"How many acres of hard drives would it take to store everyone's cellphone conversations?"
That can be answered with a few assumptions and some basic math.
First, let's say "everyone" consists of all US citizens. Let's say there are 320M citizens in the US. Let's add another 30 million tourists and/or illegal aliens bringing the total to 350M.
Now let's say everyone talks to each other 1 hour per day on average. That would total 175M hours of conversation to record per day, or 10,500,000,000 minutes. MP3 can easily store about 1Mb/minute. Therefore, it would take about 10,500Mb or 10Pb per day to store every conversation for a day. That would mean you need ~ 3650Pb to store a year's worth of conversations. That could be stored on about 1M 4 TB drives. If you can get them for $200 a pop, that could be bought for about $200M plus the hardware to park them in. I suspect you could do this where it's searchable, raided, and usable for well under $400M and store it all in a medium sized facility. It would take about 4000 racks to store it all, but that could be a 62x62 configuration. To answer the original question, this could all be stored on a single acre.
In reality, you would need a lot less space. Everyone doesn't have a phone. Everyone doesn't talk on the phone for an hour. I suspect the average is 1/10 of that. I'm doing good to talk an hour a month.