I think you are just asking how to create a Variable product…
– create a new product, choose “Variable” for type
– under attributes section, add a new one, and call it “Size”. Under the values for this attribute, type S | M | L | XL | 2XL
check the box “use for variations”
– click Variations tab, and then add a variation for each of the sizes. You can then assign each one a price, and a quantity available.
Once you have that product set up, in future you can just expand all the variations and update quantities easily.