There's no separate quantity, size, or color field on a wish. Write those details into the wish's Notes or Description instead, so gifters know which version you want.
Steps
Open the wishlist and select the wish.
Add the specifics to the Description or Notes. Something like "size M, navy blue, two of them" works.
Save your changes.
Good to know
Be exact about the variant. If the product page shows several options, name the one you want: "the 500 ml bottle, not the travel size."
A wishlist isn't a cart. Saying "two of them" in the notes is a request to your gifters. Wishpicks doesn't sell or count items, and people still buy from the original store.
Notes and description are both visible to gifters, which is the point. Use them to spell out what a link alone can't say; see "What should I write in a wish description?" Setting a priority helps too, so gifters know which wishes matter most. See "How do I add notes, priority, or details to a wish?"
Editing a wish on a shared wishlist needs Contributor access or higher. See "What is the difference between viewing and editing access?" in the Shared Wishlists collection.