Bakugan Size
For the whole life of the franchise, Bakugan toys have been made in multiple sizes. This page lists all of the existing size variations in Bakugan.
B1
28mm Bakugan, usually unofficially referenced as "B1" in reference to the name of 32mm "B2" Bakugan, were the original size Bakugan started in. Only twenty-six Bakugan variations ever got printed in 28mm size, specifically Series 1 and Series 2.
B2
32mm Bakugan, or Bigger Brawlers (B2 for short) came in replacement to the 28mm Bakugan starting with the BakuPearl series to allow more advanced engineering and higher durability. B2 was the standard size of Bakugan for the majority of Generation 1, but was ultimately replaced by the 36mm size for the same reasons of added complexity and durability in Generation 2.
36mm
36mm Bakugan were the third standard size of Bakugan. First, they appeared as "Sabu Roku" Bakugan (which stands for "three six" in Japanese") in Japan, and in BakuBlaster series of Mechtanium Surge.
Deka
Deka Bakugan are 100mm, enlarged versions of normal Bakugan. Deka Bakugan are meant to capture all the details in maximum accuracy, which is sometimes is not a possible option for normal sizes.
Nanogan
Nanogan are small, 24mm Bakugan with no pop-open feature, that serve as an extra game piece. They got introduced in Bakugan: Evolutions in EV set.