Here is the list of available formats:
Photo Book A4 Portrait (29.7x21cm) (soft cover/spiral/stapled/hardcover/layflat opening)
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Photo Book A4 Landscape (21x29.7cm) (soft cover/spiral/hardcover/layflat opening)
Small Square Photo Book (21x21cm) (soft cover/spiral/stapled/hardcover/layflat opening)
Photo Book A5 Landscape (21x15cm) (soft cover/spiral/stapled/hardcover/layflat opening)
Photo Book A5 Portrait (15x21cm) (soft cover/spiral/stapled/hardcover/layflat opening)
Large Square Photo Book (30x30cm) (soft cover/spiral/hardcover/layflat opening)
All these formats are accessible by clicking on the menu tab at the top "I create my photo album"