Warning: This is 3.0 Edition material. It is possible that there is more recent 3.5 Edition version.
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Vermin Wild Shape

( Epic Level Handbook, p. 68)

[Epic, Wild]

You can wild shape into vermin form.

Prerequisite

Beast Wild Shape (EL) , Knowledge (nature) 24 ranks, wild shape 6/day,

Benefit

You can use your normal wild shape ability to take the form of a vermin. The size limitation is the same as your limitation on animal size.