I think you're going about it in reverse order. Start with the main tanker table,that contains a unique ID, name price and any other static info for each item. Then create the table for the sizes, also with a unique ID, and another table for colors, again with a unique ID.
At this point they are separate entities and aren't related until a use configures a particular T shirt.
On the selection page, list the available t shirts along with the selectors for size and color. I would use drop downs and populate from the database tables. The drop downs would display the size and color, but the value would contain the record ID. When submitted, insert into an ORDERS table that contains just the IDs for t shirt, size and color. The orders table will have it's own ID to serve as the order number.
If you also capturing user info, include the user ID. The relationship comes not play by the orders table and you can construct a query to display the data.