This is necessary if you want your customers to select a size, colour or other attribute of a product before placing an order. Each attribute can be the same price, but you can make separate prices for some attributes.
First, under WooCommerce, Attributes, add an attribute for each group of (in this case) sizes.:
Edit each product and change Product Type to Variable:
Attributes: Select an attribute with the group of sizez for this product:
Tick Used for Variations and select the sizes (Select All)
Save before going any further…
Select Variations and click Add Variation:
Expand, then leave “Any Sizes” (in that group) and add the price. (You can select individual sizes and add different prices for each).
Save at that stage…
Then publish the product…