1) To add filter by size use WooCommerce Filter products by attributes widget http://prntscr.com/snkwjl and choose the size, then it appears in the shop page sidebar http://prntscr.com/snkwb7. There is no other sort by size option than filter widget.
2) To show the sizes on the single product page if this product is simple you need to enable tabs http://prntscr.com/snkxr5 and actually an Additional information tab will display this info http://prntscr.com/snkxio
