The size of the tip used for cake decorating greatly depends on the desired outcome and technique being employed. There are various sizes available, ranging from small to large diameters. For fine detail work or writing on cakes, smaller tips such as #1 or #2 are commonly used. Medium-sized tips like #12 or #16 are versatile options suitable for creating borders, flowers, or piped designs. Larger tips such as #18 or #21 are ideal for filling large areas or adding bold accents on cakes. Ultimately, the choice of tip size depends on the design and effect you want to achieve in your cake decoration project.