Boleys can be pretty large, as seen in the photo below. There are also those that are about the same size as a G3 pony.
They vary in color, some have cutiemarks. So far, all I've seen has the typical rainbow-themed mane; blue, pink and yellow.

Boley with G1 Applejack for sizecomparision. This one has had her tail and mane cut.
Image by RinLin