Drop down menu option for additional date

Nearly everyone I know who uses DSP also utilizes t-shirt size for merchandise or registration t-shirts. I have an additional required field for t-shirt size as a part of our online registration. Being able to add a drop down menu of sizes would greatly reduce the incorrect sizes and extra merch I need to purchase as parents input a vast variety of answers. For example, I have Youth S, M, L, and adult S, M, L, XL but parents enter sizes such as 7-8, 7X/T, etc. This costs me several hours of reaching out to parents to clarify, asking teachers what size they think will work, or just guessing and ordering extra shirts in different sizes and having a surplus at the end of the year. A drop-down menu means parents would select from the sizes each studio/teacher sells and would provide clarity and save so much time! I appreciate any assistance with this!