how do I edit more product information in one link

Not sure exactly what you are asking, or what listing tool you are using, but to have a listing with multiple sizes available and to allow the buyer to choose a size from a dropdown menu, you need to be using either Trademe's Tradevine, or another listing tool to create what's called a Multi Variant Listing (MVL).

See this Help page for more info

Tips for professional sellers | Trade Me Marketplace

I tried choosing to use tradevine and after editing the variant info and saving it, I was actually charged?

And what was the charge for as itemised in your Trademe account transactions page? Tradevine is a tool that requires a monthly subscription/charge.

Do you mean that I can't use MVL tools without paying for a tradevine subscription?

Tradevine is a paid service that offers MVLs. You can write/use your own API application to use MVLs if you have the skills to program.