Rating of diodes for relay coil spike suppression
The following relates to interference suppression caused by the relay coil.
The relay contacts may also need 'suppression' - see at end. 

The maximum current that will flow in a protection diode is the current flowing in the coil at switch off. So a diode rated at Icoil max will be more than adequate. If the relay is de-operated only occasionally a diode rated such that turn off current is somewhere between the diode's continuous and surge current ratings may suffice.

This Digikey search lists relays rated for 100A DC or more at 12V.
The maxumum coild current is 3.3A for a Panasonic  300A switchingrelay
and under 500 mA for a superb OMRON GPEA-1 relay rated to switch 100A at 120 VDC, and 60A at 400 VDC.

An inductry standard 1N400x diode is rated at 1A continuous, 10A for 1+ seconds and 30A one off surge (needs a nice strong cup of tea and a sit-down before repeating). So a 1N400x duode would work without question for the 500 mA coil current relay and almost certainly for the 3.3A coil current relay. 

A spike suppression diode should be mounted as close electrically and physically to the coil as is reasonably possible. Ideally across the relay or socket terminals. This minimises reradiating loop area for EMC and minimises inductance in the diode current path which might otherwise allow very short period spikes. 

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Relay contacts and switching mechanism for a properly rated relay switching DC will usually be designed to allow a minimum of arcing at the contacts. Voltage spikes will still occur if any inductive load is present and a suppressor to deal with this may be needed. This may be a di

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