What creatures can be improved familiars?

Question

What are the rules for picking something from Improved Familiar in Pathfinder? How do you know what familiars are available — is it only the ones listed on d20pfsrd.com or are there rules that let you pick your own familiar from any monster in the book?

The only rules they list on d20pfsrd is that it has to be compatible alignment and have to be a sufficient level but that's it. I thought there was one more saying your familiar can't be as tall or taller then your character, so if you're medium your familiar has to be small or smaller. But I can't find that final size restriction rule anywhere.

Answer 1

A creature is limited to the improved familiars the feat lists and improved familiars listed in later texts. The feat Improved Familiar says, in part, that

When choosing a familiar, the creatures listed below are also available to you. You may choose a familiar with an alignment up to one step away on each alignment axis (lawful through chaotic, good through evil).

The feat then provides a list of familiars. Later books add to this list of improved familiars (that's the bigger list found on d20PFSRD.com; the above link goes to the official Paizo site).

The feat Improved Familiar does not allow picking any creature as an improved familiar, but the master's size and the familiar's size are not factors when picking a familiar, improved or otherwise.

Dungeons and Dragons 3.5 in its Dungeon Master's Guide contains rules for Alternate Familiars by Master Size (203-4), which may be from where this confusion stems; such rules appear to be absent in Pathfinder.