If I choose color-size for variations I am able to enter color and size, but now I also have to fill out a “Shirt Size Value” field which wasn’t there before.

I like a puzzle, so took a look at this even though I don’t sell apparel.

This is what I got from what I read.

“Size” and “Shirt Size Value” are the same thing.

In the old way (Size), you used to be able to type whatever size you wanted. In the new way (Shirt Size Value) you’re going to be forced to select it from a list. During the transition from old way to new way, both fields will still be visible, while sellers get used to choosing their sizes from a list instead of typing them manually.

I got that from this statement -
Will the existing “size” attribute still be available for listing experience?
Post the enforcement of new attributes, we will remove the existing “size” attribute from the listing experiences (Translation: Once everybody learns to do it the new way, we’ll remove the ‘size’ box completely). During the transition phase when both existing size attribute and new standardized apparel size attributes are available (ie both will be available to confuse the heck out of everyone while we transition, because we are Amazon and we believe that making things complex, confusing and convoluted is best), the values submitted through the standardized apparel size attributes will take precedence to generate the customer facing size value. (Choose the right items from the Shirt Size Value list because we’ll ignore what you enter as ‘Size’)

That is taken from the ‘Apparel Size Standardization Seller Guide’ found here -

So it seems like you should enter identical sizes for both Size and Shirt Size Value, then delete any variations that don’t match, like ‘Green Medium Large’. A listing for ‘Green Medium Medium’ would mean - the shirt is green, and done the old way, the size would be medium, and done the new way, the size is still medium. I think Amazon will then assume the shirt IS actually size medium.

Enter the sizes as you normally would in the ‘Size’ variations, but understand that ‘Size’ will eventually go away and only ‘Shirt Size Value’ will remain. You enter the exact same data (shirt size) for this field, the only difference is you’ll be forced to choose your sizes from a drop-down list rather than type them in by hand.

@EtoCart

but I still wish their was some updated guide specific to clothing listings.

There is. Read the PDF file above, and also check out this help page -
https://sellercentral.amazon.com/gp/help/GM6YJ5XPA58U99XM?language=en_US&ref=ag_GM6YJ5XPA58U99XM_cont_home