in terms of height they're pretty much like any standard book. but for width, they're not harry potter thick. the novels are around 150-180 pages. some may be a little longer.

if you want recommendations, i suggest getting the one i linked you and this one. those two characters (shikamaru and kakashi) are fan favorites so your son probably also likes them

For anyone interested, kakashis story is like a bit less than 5 bucks on Amazon and it's prime friendly Naruto: Kakashi's Story https://www.amazon.com/dp/1421584409/ref=cm_sw_r_cp_apa_z2U8yb8Q0G95B

Kakashi's Story

http://www.amazon.com/Naruto-Kakashis-Story-Masashi-Kishimoto/dp/1421584409/

http://www.amazon.co.uk/Naruto-Kakashis-Story-Masashi-Kishimoto/dp/1421584409

http://www.barnesandnoble.com/w/naruto-masashi-kishimoto/1122090177?ean=9781421584409

http://www.bookdepository.com/Naruto-Kakashis-Story-Masashi-Kishimoto/9781421584409

Shikamaru's Story

Released in February

Sakura's Story

Released in May