The block size? The block size is most definitely not going to by 700 Gb. That's way to large. You're probably thinking of the blockchain size. The blockchain currently grows at a rate of 144 MB per day. With Segwit, it can grow at a maximum of 576 MB per day. You can then extrapolate that to the maximum size the blockchain at any point in time.
What happens once the size goes beyond 1TB, or beyond the biggest HD? what happens when you can't fit the blocksize in a regular HDs? Can you run 3/4 of the blockchain then the remaining 1/4 part in another HD?
Hard drive capacity will continue to increase, and it will most certainly outpace the growth of the blockchain. Disk space gets cheaper by the day. The concern should really be on bandwidth, CPU, and RAM bottlenecs. It takes a lot of bandwidth to download the entire blockchain and a lot of CPU and memory to process all of that data.