Not strict at all on size, more on numbers. Just don't have more than two bags and you will be fine on most size planes. This is my experience as well; I can't recall seeing sizing enforced but definitely numbers.

Delta Air Lines is generally known for having a consistent but somewhat flexible approach to carry-on luggage compared to budget carriers, though they have become more vigilant in 2026. Their official policy for carry-on bags is a maximum size of 22 x 14 x 9 inches. While they do not strictly weigh carry-on bags on most domestic routes (except for a few specific international locations like Singapore or Beijing), they do utilize "sizer" boxes at the gate, especially on full flights. If your bag is visibly overstuffed or clearly exceeds the dimensions, gate agents will likely require you to gate-check it. However, unlike ultra-low-cost airlines that charge a premium for this at the gate, Delta often offers to gate-check bags for free if the overhead bins are reaching capacity. For checked luggage, Delta is much stricter; any bag exceeding the 62 linear inch limit or the 50-pound weight limit for Main Cabin will trigger significant oversized or overweight fees, which can start at $100 and go up depending on the route.