Three formats of the same simple object, and the thing that decides between them is length. A bookmark that does not show above a closed book cannot be seen or grabbed, so it stops being used within days, which is the only way a promotional bookmark actually fails. Filter by how it will be handed out.

Full length. It must show above a closed book.

2″ × 6″, volume giveaways and mail inserts.

Shaped or non-standard: a shape, a clip or a matched size.
Length is the whole decision. A full-length bookmark protrudes clearly above a closed paperback, which is the entire functional requirement. It can be seen on a shelf and grabbed without opening the book. The compact 2″ × 6″ format shows above most paperbacks but not larger hardbacks, and it exists because it costs less per piece and fits a standard envelope without folding, which makes it far easier to distribute in volume or include in a mailing.
Two small decisions matter more than they look. Lamination is doing structural work rather than decoration here: a bookmark is slid in and out of pages hundreds of times, and an uncoated one softens at the corners within weeks. Its entire value is that it stays in use for months or years. And rounded corners cost almost nothing while removing the exact place a bookmark first curls. If the design needs a shape, a clip that cannot fall out, or a size matched to a specific publication, that is what the custom format is for.