Please share your store URL and screenshot.
So that I will check and let you know the exact solution here.
So basically rather than the drop-down box for size I would rather all sizes being displayed next to eachother
Look I have checked your requirement, this is the time consuming task,you have to do code customization work.
We are available for your work If you wish !
If you Want to modify or incorporate custom changes on store, Hire us, We are having expert shopify developers
Please you can contact me on this
Skype : oscprofessionals-87