@ism-freelance
I would imagine that you would add a drop down to your page, where you would place the relevant size options. You can either hardcode them via the drop down settings, or read the “productOptions” field from the product and then set the values via code.
Either way, now, when you press the addToCart button you would need to read the value from that drop down and then set the value under “choices” within the addToCartOptions.
Finally, you might want to change the price displayed on the page upon a selection made in your drop down, and I suggested ways to handle this above.
I hope that’s clearer, if not please let me know.
Thanks,
Oded