How can i solve this issue "show product quantity in stock respect to size"

Hello,

Please change the product type to the variable product. Then you can manage stock for your each variation. You can check this screenshot – https://prnt.sc/akokq3. Now, click on the attributes option and add your attributes- https://prntscr.com/akon6n.

Now after adding your attributes now create some variations with your size or color- https://prntscr.com/akonww. In the screenshot, you can see the small size variation I have inserted the stock.

You can add the stock and different price for your each variation.

Also, you can read this doc- https://docs.woothemes.com/document/variable-product/.