The IB3600 is the smallest as mtveten said. A little longer than a GP3300-but a smaller diameter can.
The IB3600 is soon to grow 0.2mm in diameter. This is a technical change to allow for more room for the internal componenets of the battery. Theres more "stuff" packed in a IB celll than a GP3300-so its been a tight tolerance thing.
The GP3700 is a bigger, heavier cell. The IB3800 willl match the GP3700 in exterior dimensions very closely.
IB3800 will not only be longer and bigger in diameter than the IB3600-but it will also be heavier.
Hope this helps,