The parent poster, as an outsider, only makes informed, but speculative guesses about Twitter's business footprint, as does ~anyone who does not have T9 vision of the firm.

I am not going to tell you that their right size is 3500, or 1500, or 7000, or 14000, because I don't know what it is. But I will say that it's very likely to be measured in the thousands. [1]

[1] Although you can always try to shrink the business... A Twitter that doesn't work, and has no revenue could probably be ran by ~0 employees.