What criteria should I use to choose between the two headboard sizes?

You choose between two width specifications^[1]. munito's POISE BED features four body options—Q (Queen), K (King), LK (Large King), and SK (Super King)—and the headboards that pair with them come in two types with different widths^[2]. Depending on which head you pair with the body, the proportion of the headboard margin remaining on both sides of the mattress will appear differently^[3].

The frame body is also not made to fit the mattress tightly. It is designed to leave extra space like a border along the left, right, and front, even when the recommended mattress is placed on it^[4]. If there is space between the mattress and the frame, your hands can easily slip in without having to lift the mattress high when changing sheets or bedding, and it also provides a stable support when you briefly perch on the edge upon waking up.

In the end, choosing a headboard is a matter of deciding how wide to make the margins on either side of the bed. If you choose a head wider than the body, the headboard fills the wall like a hotel bed and anchors the room, whereas if you match it closer to the body width, circulation along the walkways on both sides becomes more spacious.

hmmm, not locking the head to a specific body one-to-one is closer to a suggestion to look first at the left-and-right proportions of the entire bedroom wall rather than just the mattress size.