The size of the mason jar for overnight oats can vary depending on personal preference and serving size. Here are some options:

• 8-ounce mason jars: These are a popular choice for single servings and can be filled all the way to the top.
• 16-ounce mason jars: These are also a good option for single servings and provide more room to stir everything together before eating.
• Wide-mouth jars: These make it easier to stir in toppings and mix-ins.
• Plastic containers: These can be more durable than glass and are a popular choice for families.

Ultimately, the size of the jar will depend on how much you want to make and how much room you need for mix-ins and toppings. Its important to choose a jar that gives plenty of room for a hearty portion of oats and mix-ins, so anything from 8 to 16 ounces is ideal for a single-serve jar.