Bakugan Size
For the whole life of franchise, Bakugan toys did come in multiple sizes. This page lists all of existing size variations in Bakugan.
B1
28mm Bakugan, usually unofficially referenced as "28mm" in callback 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 in BakuPearl series to allow more advanced engineering and higher durability. B2 was the normal size of Bakugan for the longest time, but then got replaced by 36mm size for same reason it once got introduced.
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. Later on, in Reboot series, 36mm size became standard for Bakugan.
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
Nanogans 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.