"all other things being equal", how does one tell what weight, say, a
given cylider can LIFT, straight up? for sake of discussion let's assume
load to be lifted has perfect "zero friction" bearings guiding it. I
assume the answer has a lot to do with size of the cylinder *piston*,
and amount of pressure pump puts out "to" the cylinder ? but what's
the formula?

Cross-sectional area of the piston in square inches times pressure in psi
sq.inches times pounds divided by sq.inches = pounds lift

also, can -ALL- hydraulic cylinders be operated 'in any orientation'?
eg: with cyl body horizontal, vertical, or at any angle anywhere between
the two?

Not the ones that are actuated by a lever/piston mounted to one end of the
cylinder, otherwise yes.

for application in a tiltbed trailer, I assume a cylinder that's bigger
and with a LONGER stroke mounted, say, closer to the hitch, would be
-vastly- better than having a shorter cylinder with an even bigger bore
mounted closer to the axle (so it had a shorter stroke). that be
correct, then?

Go look at one and copy their design.

You could certainly spend $20 or so on an engineered set of plans ..

GWE