How would you then say "everything is big"? Alles ist Grosse?
Alles ist groß
When you write Grosse with capital letter it means a noun. To say "everything is big" you need an adjective so - groß.
Now wait. I'm confused. Why the e ending on Große?
"Die Größe" is the noun and it means "size", "groß" is the adjective. It depends on the gender of the noun that follows "groß" if you need to use "groß" or "große".
So how then might you say, "all sizes are too big?" Alles Größ/Grosse sind zu groß..
Alle Grössen sind zu gross....( passen nicht if referring to clothes)
Why is there an umlaut? I thought groß only had an umlaut when out was used on the comparative?
The adjective big "gross" only has the umlaut in comparisons.
This is the noun "Grösse" which means size. This is where I am glad that German nouns are capitalized.
So this is nominative, but can someone explain to me why alles is used?
Because "alles" means "everything". That's just the way it is. "Katze" = cat, "alles" = everything. Perhaps you're confusing it with the word "alle" which means "everyone" and has nothing to do with "alles"
Why bigness is wrong and greatness is correct?
Bigness is not an English word. This is a noun "Grösse" which means size. It is not to be confused with the adjective "gross" which does have the umlaut in comparisons.
Could "Größe ist alles" also be used as an idiomatic way to say "one size fits all"?
No ..thats not what it means in german...one size fits all....would be..... Gleiche Grösse passt Allen....nobody says that though....;) one size fits all is well understood.