what`s the difference between a small-shank mouthpiece and a large-shank mouth piece. I wanna get that 5g mouthpiece from mouthpiecewizard.com but idk if it`ll fit in my trombone. By the way its a Yamaha YSL-691

Small shanks are for smaller bores, large shanks are for large bores. You are playing on a great lead horn. It was designed to be played with a small mouthpiece, and to play high notes. If you want a 5G because you want a bigger darker sound, you may want to look into a different trombone. What size mouthpiece do you play, and what do you want to change in the way it sounds?