すみません、マグロをシャリ小さめで握っていただけますか？ — Excuse me, could you make the tuna sushi with a smaller portion of rice? (Sushi · Restaurant, Formal — to staff, JLPT N4)

Gesture & etiquette

Slightly bow your head and make a small gap between your thumb and index finger to indicate 'small'.

Do not leave the rice behind and only eat the fish toppings; this is considered very impolite in Japan. Always ask for 'shari chiisame' beforehand if you cannot eat much rice.