Here's a showcase of all the Zentangle, Apprentice, Zendala, and Bijou tiles that I've done.
Zentangle tiles are 3.5" square. Apprentice tiles are 4..5" by 4.5". Zendala tiles are circular and of 4-5/8" (117mm) in diameter. Bijou tiles are 2" by 2" of cuteness.
I am a Certified Zentangle Teacher and teach Zentangle classes in Singapore. For more information about that, click this link.