I've heard that silver dollars can get as large as dinner plates(?!) so I know they need a large tank but how large do you think 6 adult silver dollars would need? There would be other tankmates, but nothing too large except for the SDs.

And does anyone know how fast they grow? E.g. would a 90 gallon hold them for a couple years or no?

Some silver dollars do get very large such as the "red hook". Others, like the spotted variety stay relatively small, 3" to 5". I have 11 of those in my 120. Most of them are about 5 years old. The biggest is a female, approx 5 inches. I believe their scientific name is "maculatus" spelling?
8 of them grew up in a 65 gal. Community tank and did just fine.
Your 90 gal. Tank should be fine for a school of 6 to 10 SD' s provided you purchase the smaller varieties.
It's also good to provide 2x the rated filtration for your tank. I run 2 fluval/Aqua clear 110 filters on my 120. I also change 20% of the water every week.
Don't worry too much about the water chemistry. Most SD's are farm raised and have evolved to adapt a wide range of water perameters.
SD' s are hardy and tough. Mine live well with a green terror cichlid several red terror "Festae" juveniles and various african rift lake cichlids. I have kept SD' s in my tanks for many years; one of my favorites.

I was really interested to hear this about the spotted silver dollar. I've never heard of those before and I've always loved silver dollars but I can't have a large aquarium for the foreseeable future, which is why I don't have the large kind of goldfish or giant gouramis. So I didn't think I could have any silver dollars until now.... and now I'm already looking into having some spotteds lol.