How do I select diodes for battery charger repair?
Some how I missed where you wrote that the charger was rated for six amperes.
That's the answer to your question.
You need diodes rated for at least 6 amperes.  Anything above 6 amperes is good.  If you have doubts, look at the &quot;ampere&quot; scale on the charger and use diodes rated for the maximum current the meter can display.
There are 10 ampere diodes available from various places that should do just fine.
I find 10A10 diodes cheap on non-specialist sites.  They are rated 1000 volts and 10 amperes.
You might also consider replacing all the diodes with a bridge (something like an NTE5322, 25A 200V)  that you can screw to a metal part of the charger housing.  The advantage is that the metal helps keep the diodes cool so they last longer.

Some of the switchable 6/12V chargers worked by switching from full bridge (12V) to half bridge (6V.)  That may make it more complicated to use a bridge rectifier in a single package like the NTE5322.