Is it possible to find out the key size of the signer if we only have the signature

[Weijun Wang]

I am writing a tool to warn about weak key usage in a certificate or 
CRL. One of the warnings is if it's signed by a cert with a small key size.

But the signer's cert is not always available. I can see that the 
signature's size depends on the signer's key size. Is there a reliable 
way to calculate this key size? The only existing knowledge is the 
signature bytes and the signature algorithm.

Thanks
Max