As BM says, it depends on the material you're drilling. Let's assume mild steel.
The drill bit manufacturers pretty much answer your question - on a 2MT, drills normally go up to almost an inch for instance. There's always someone who'll manufacture something outside the norm, but the standard ranges are a pretty good guide. Eyeballing it, a taper will handle a drill about 30% larger than it's thick end in steel (no science behind this - it's just what I've seen over the years).
An inventive HSM'er can certainly go beyond this; drill in steps, add a torque arm to the drillbit if in the tailstock, slow the spindle speed down, many ways.
I have a 2 1/2" drill on a 5MT, and with a 1/2" pilot, it sails through mild steel. If it had a 2MT, I'd expect problems - spinning in the taper, ripping the tang of etc.
What tapers do your machines have?
All of the gear, no idea...