If you want a limit on familiar level, there's a precedent in TMRE, page 82. For binding an aspect of a Daimon: "As a rule of thumb, if the (Technique + Form) proposed for the bond is less than 2 x Might, it is likely to reject the magus as not worthy." You could apply that to other familiars as well if you want. But you'd probably want to apply a Size modifier as well.
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That is a good guide. I like it