A saddle that's the wrong height is one of the most common — and easiest to fix — reasons a bike feels uncomfortable, even when the frame size itself is correct.
Sit on the saddle with a heel on the pedal at its lowest point. Your leg should be almost fully straight, with just a slight bend. When you shift your foot to the normal pedalling position (ball of the foot on the pedal), that slight bend becomes the correct amount of knee flex at the bottom of each stroke.
Too low, and you'll feel like you're pedalling with your knees — less efficient and harder on the joints over a long ride. Too high, and your hips rock side to side to reach the pedal, which is both inefficient and uncomfortable. Loosen the seatpost clamp, adjust in small steps, and re-check after each change rather than guessing the whole distance at once.