For RSA keys, is the safety of a given key-length different for signing vs encryption?
No, the security is about identical, as the underlying RSA problem is the same.

Besides that, non-repudiation is usually managed differently with regards to legal requirements. Providing confidentiality (encryption) is a rather different use case than non-repudiation (signing a contract).

It is also possible to use signatures for authentication rather than non-repudiation. In that case you may be able to get away with smaller signatures if the authentication and key is only valid for a limited amount of time.

All in all, the key strength should be considered given the use cases, threat model and of course the system parameters. Defining a minimum RSA key size without considering the system is not a good idea.



Notes:


It could be possible to assign a notary that immediately signs the document after signature generation by the other party. The notary should of course use a key of sufficient strength.
I've used the notion of a larger key, but what you're really after is a stronger key. Usually larger keys are stronger for the same algorithm though.
If you go over a 16Ki key size then the key size doesn't matter much anymore. The key will only be broken if RSA itself is broken (e.g. using a hugely powerfull Quantum Computer).