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For sizing, like @Lyle K justly pointed out: each component on the loop is sizing to meet the full loop load multiplied by the component-level sizing factor. See for example for a Chiller:Electric:EIR

The full loop load is calculated by what's on the demand side of the plant loop, so in general that's driven by the zone cooling loads or airflow (for a coil placed on an AirLoopHVAC). Both are affected by the Sizing:Parameters Cooling Sizing Factor.

So what happens for a chiller really amounts to

Full loop load * chiller Sizing Factor  * global cooling sizing factor

So if you want two equally sized chillers, and a global cooling oversizing of 15% you can:

Create two Chillers, each with a Sizing Factor of 0.5
Assign 1.15 Cooling Sizing Factor on the Sizing:Parameters object

You can see that this is what openstudio-standards does for eg around here

(Note that you could also do sizing parameters of 1, and 0.575 sizing factor for each chiller... depends on what behavior you want for other objects... for appendix G, do what's above)

Sizing Factor allows the user to size a component to meet part of the design load while continuing to use the autosizing feature.

So what happens for a chiller really amounts to

Full loop load * chiller Sizing Factor  * global cooling sizing factor

So if you want two equally sized chillers, and a global cooling oversizing of 15% you can:

Create two Chillers, each with a Sizing Factor of 0.5
Assign 1.15 Cooling Sizing Factor on the Sizing:Parameters object

You can see that this is what openstudio-standards does for eg around here

(Note that you could also do sizing parameters of 1, and 0.575 sizing factor for each chiller... depends on what behavior you want for other objects... for appendix G, do what's above)