I believe that the fuse should be the same size as the amperage of the charger. 35 amp charger, would therefore use a 35 amp fuse.
Here it is directly from from the ABYC:
A-20.5.6 Each ungrounded DC output conductor shall be provided with overcurrent protection, based on the maximum output of the charger, within the charger, or
A-126.96.36.199 the charger manufacturer shall provide instructions to install the overcurrent protection device within 7" of the charger.
EXCEPTION: Self limiting battery chargers.