So you want 15GB/s of throughput, and 50TB of capacity, and you want to do it cheaply? Well, I guess that depends on your definition of cheap.
Have a look at this Cost Per Petabyte (about 1/3 of the way down the page). Now, that's of raw SATA storage. SATA is not fast. It's certainly not 15Gb/sec fast. So feel free to, say, double or triple those costs (before you get excited about backblaze, the IO on those boxes is going to be rubbish. They stream their data out over the internet and probably don't even need to saturate a 1Gb/link for each of their arrays).
All disk arrays of this calibre are 19" rack mountable. All of them. And what is "small"? 3U? 30U?
Your other challenge is getting 15GB/sec of throughput. 15GB/sec is 120Gb/sec, and given that the fastest interface I've seen on a single disk array is 10Gb/sec, you're going to need a minimum of 12 arrays to get that kind of throughput. That's assuming that a single array can saturate a 10Gbps link. At 3U per array you're already up to 36U, or almost a full rack.
This is totally unofficial, but based around some pricing that we recently received:
12x 16 Disk storage array (10Gbe, 16x600Gb 15k SAS drives configured in RAID-10) will roughly get you what you need. That's accounting for redundancy and whatnot. They're 3U and let's say $50,000 each. That's $600,000 and 36U of space. Plus the costs of the rest of the 10Gb infrasturcture, expect to be spending closer to $700,000.