How to add "Size" option in my Commerce Product without adding lot of SKU that differs by size?
You're going to create 5 option set.


White with sizes
Blue with sizes
Green with sizes
Orange with sizes
Yellow with sizes


Each Option set create you created insert your size field (list type)

On your Shirt product type or whatever your product type is, create your sizes option set reference field

So when you create your 5 SKU you're going to choose which option set the user is going to use.

For Example:


Woman's T-Shirt WHITE (PRODUCT) 
Option set reference:"White with    sizes"