This pattern is named for Polystichum acrostichoides, also known as the Christmas fern, because of the cable design on the center back of the mittens.
It’s an adaptation of Knitterkaren’s Comfy Gusset Mittens (link above), just adding some embellishment to the plain back.
The sizing for Acrosti is generally the same as the original, however this pattern only serves Adult Medium. Despite this, the gusset is wide and comfy, and should be flexible sizing-wise.