When Is it appropriate to ask user to choose apparel size ( XL,XXL) ?on the product screen or on the cart screen?
I'd test it to be sure.
However my hunch, having worked on a fair number of clothing ecommerce systems and having dong testing on those, is that you want to be specifying size on the product screen, or before (via search by size for example).
The reason? Almost every clothing store will not have every single size in stock for every line of clothing. Certain sizes will tend to sell out on a line before others.
So - if you have specifying size late in the process the experience of shopping for a bunch of users becomes:
This happens too often - folk just give up.
Assumption: You are following the amazon-ish way and product selection on product screen leads to cart/check-out screen.
When the user is selecting the product, you cannot expect them to purchase a garment without specifying its size. It might lead to the feeling that there is no size selection at all.
eg: how would you like to walk into a shop and pick out a t-shirt to buy and you can only see the size when you reach the cashier?
The cart needs the size display for re-inforcement that the user picked the correct size and also allow for change incase the user decides to change it.
The product screens.
That way you can show what is available and in stock, if you've created your site to be able to do that.
Also on the listing pages size is a good filter and you can also provide information on the listing pages as well when users interact with individual items.
The cart screen is far too late for most things.