US airlines - Questions from an Australian

G'day brains trust, I'm travelling to the states in july and am wondering how strict I can expect the airlines to be? I'm bringing a a Tortuga outbreaker 35l and my mate has the Kathmandu litehaul carry on 38l. Will we be asked to check these or be asked to pay more as they're a little larger than your average backpack? Flying with delta, United, jet blue...