1) By at least an order of magnitude larger than all other countries combined.
2) All infectious diseases you can think of. By the end, they had begun to engineer chimera viruses (Smallpox and Ebola for example). Means of deployment everything from spray tanks to artillery shells to ballistic missiles.
3) Hopefully none. Bioweapons have a nasty tendency to bite their deployers in the ass. They do not respect borders and the Soviet military was notorious for its poor safety standards (that was an understatement). Viruses also have a nasty way of mutating into something potentially even nastier, even without the radioactivity of a global thermonuclear war.
4) FUCK NO!
Biowarfare scares the shit out of me, a LOT more than nuclear of chemical warfare. The Soviet biowarfare program was an especially ugly case of "we are doing it because we can".
NOBODY needs chimera viruses, except for bragging rights. They were also elated at the elimination of Smallpox, because it gave them a monopoly.
Even the USA's biowarfare program in oh so infamous Ft Detrick was a fart in a high wind in comparison. The Soviet biowarfare program (which still exists in Russia, just sayin') was straight up pure evil.
For a nice introduction, I recommend "Biohazard" by Ken Alibek and work from there. It gave me and still gives me the heebie jeebies.