I should clarify, though, that if you yourself cast _permanency_ on yourself, the spell can then only be dispelled by someone higher CL than you (no matter what they roll on their dispel check). Which means that, yes, if you yourself have _permanency_ prepared, it's worth it to get _enlarge person_ from a scroll or from a low-level caster, as your CL overrides his.
However, this wasn't the OP's question, and if you're capable of casting 5th-level spells, you're unlikely to be the kind of character who'd find being permanently enlarged an advantage.