I’m not sure whether this will be helpful data or not . . .
I once downloaded all the Apple II software that’s available on the various archive sites dedicated to preserving such things. In total, these files occupy 3 CDs, or about 2 terabytes’ worth. This was a few years ago, maybe 2003, but I doubt the archives have grown much since that time.
Now that figure, 2 TB, will be an underestimate. It won’t, or shouldn’t, include much of the commercial software — not legally at least, though you won’t be surprised to learn that several commercial titles made it into the archives anyway. And it won’t include private software projects that never saw public release. The archives do however seem to include most everything ever released in magazines, or what was kept on bulletin boards — which is how freeware and shareware were distributed back in the day.
The OP asked about DOS software, not Apple II, so I won’t work too much harder trying to nail down a number. The Apple II was a smaller machine, and would tend to require smaller programs, but on the other hand it was actively used and sold for about the same time period as MS-DOS.
There must be archive sites out there dedicated to preserving old DOS software, as there are for the Apple II, the C64, the Ataris, and even the Commodore PET. (No terabytes needed for that one!) So maybe take the size of the biggest or most popular site, multiply it by 2 or 10, or π, and you’d get a rough estimate of everything that was ever available for DOS.