According to the Scrivener manual for Windows (I hope that’s correct, I’m a Mac user) you’ll find the setting via Tools → Options… → Corrections: “Fix capitalization of sentences”. And maybe also (don’t) “Capitalize ‘i’”.
Scrivner should NEVER start out with autocorrect capitalizing the letters after a punctuation mark. I say this because it is a writer’s program. Writers write dialogue. You don’t capitalize the letter following the dialogue.
Actually, it’s more complicated than that. If the line of dialogue is followed by a dialogue tag, then there’s no capital letter:
“Watch out!” he shouted.
But if it’s an ordinary sentence, it should be capitalized.
He shouted, “Watch out!” The men in the boat couldn’t hear him, though.
And of course writers, even fiction writers, write a lot of things that aren’t dialogue as well.
In any case, in Mac Scrivener you’ll find the option in the Settings → Corrections pane.