How much soap goes in the big washers ? I have no idea and my queen comforter ( Washable) is getting filthy looking

The oversize washer at the laundrymat I use occasionally has a sign telling you how much to use. The washer is front loading, so you don't have to use as much as you might think.

As mentioned, front loaders do not use as much detergent, so I would check with any signage or a person who works at your laundry mat. Seems many of the larger places have a host/hostess to assist.
I would also use a pre-treat on any old soil/stains well in advance of washing it, for the best results.

The laundry places I go to don't have anyone there nor any directions. I guess at the amount, going on the short side if anything. Mine has always come out good. I do pretreat the worst spots.

I asked the attendant, as I hadn't been to a laundrymat in years. If there was no attendant, I would follow the directions on the detergent and pre-spot.

If you've finished your washing project, I hope it came out fine.

Yes, if you have to guess - use less. I would even use less than the instructions at the machine state. Any soap left in after the rinse attracts soil.

I also like to add white vinegar to the wash.