The capacitors size themselves are small. They slightly vary but are almost always the size of a medium Red Bull can. They are sized to your unit though so you can’t just buy one without looking at the specs and expect it to work. But the specs are printed on the side of the can and if not can be derived from the specs on the unit itself.
As far as their danger there really isn’t any beyond getting shocked from dealing with live wires. Technically they can retain a bit of a charge so I’ve seen recommendations to wait X amount of time before touching them with your bare hands or to discharge it by touching it with an insulated screwdriver to discharge it but the risk is pretty low. Once the power is off (either at the breaker or via the disconnect at the condenser unit, power only goes in one way to those things so if you turn it off in one place there’s no way you’ll get a zap) it’s a soda can with 3 wires going into it. You just disconnect the 3 wires from the old soda can, remove it, replace and connect the new one. Not that much harder than changing a light bulb.
As far as their danger there really isn’t any beyond getting shocked from dealing with live wires. Technically they can retain a bit of a charge so I’ve seen recommendations to wait X amount of time before touching them with your bare hands or to discharge it by touching it with an insulated screwdriver to discharge it but the risk is pretty low. Once the power is off (either at the breaker or via the disconnect at the condenser unit, power only goes in one way to those things so if you turn it off in one place there’s no way you’ll get a zap) it’s a soda can with 3 wires going into it. You just disconnect the 3 wires from the old soda can, remove it, replace and connect the new one. Not that much harder than changing a light bulb.