The 3.5 feat "improved familiar" states that you can acquire familiars that are not on the standard list. It provides a table of possible familiars along with caster lvl prerequisites, alignment prerequisites, and even type prerequisites. However, it also states that your new familiar options are not limited to the provided list.
The list in the table above presents only a few possible improved familiars. Almost any creature of the same general size and power as those on the list makes a suitable familiar.
So how can I determine which familiars I can get? I have seen familiar guides and handbooks that mentioned getting hippogriffs for example, which are not at all the "general size and power" of a Shocker Lizard. None of these guides seem to have a familiar with a caster-level prereq higher than 7, aside from familiars for which you need other feats alltogether.
So, how do I determine which creatures can become my familiar due to the improved familiar feat, and what their caster lvl prerequisite is?