Thank you for reaching out to the community about this. To allow your customers to choose a size, you will need to add the sizes as variants in the product's settings in your admin. From your home page, navigate to Products > select the product, scroll down to the Variants section. If it is a new product, you will add the option "size" and then add the choices so that it looks something like this:
If you've already added other options, such as colour, then you will instead need to click on More options > Edit options > Add another option. So it will look something like this:
Once you've created that second option, you'll be able to more easily create additional variants by clicking on "Add variant", and filling in the options. You can find full instructions in our help docs on how to do this here, but feel free to let me know if you have any further questions about it!