Shopify helps merchants configure the layout of the theme with Blocks, if your theme does not support Add Block, try this method for changing the position of your size button.
These instructions are for everyone. No previous coding knowledge is required.
Step 1
To find out the CSS class, head over to any product page in your store and click on 'Inspect'
Step 2
Click on the selector icon
Step 3
Hoover over the component you wish (the button will be placed right below this component).
Step 4
Click on the component and check the code on the right pane. The relevant CSS class for this component will be selected. Copy this class (text contained between brackets ''").
In the example below: price price--large
Step 5
Paste the class in the relevant section and save the changes.
Step 6
Verify the new position in your store.