You need A5 not A6. A5 is half A4. I assume that you want a folded booklet.
Start new publication choose Blank Page sizes then choose booklets and then 1/2 A4 booklet 14.8 x 21. It will then prompt to create your 4 pages. Insert your pictures and the booklet is complete until you come to print. It is landscape and ideally should be printed with an auto-duplex printer but manual duplex works ok but involves some trial and error in my experience.