Length of ElGamal signature compared to RSA signature In his original work (1) on the ElGamal encryption and signature scheme Taher ElGamal states in chapter V. section B.:


  For  the  signature  scheme  using  the  above  arguments  for   the
  sizes of the numbers in our system and the RSA system,  the  signature
  is  double  the  size  of  the  document.  Then  the   size  of  the 
  signature  is  the  same  size  as  that  needed  for  the   RSA
  scheme, and half the size of the signature for the new 
  signature scheme that depends on quadratic forms [...]


However, I was under the impression that (schoolbook) RSA signatures have roughly the same length as the corresponding modulo n, while ElGamal signatures are roughly twice the size of their corresponding modulo p.

Could somebody please clarify?
Mistakes happen.

The author's scheme adds a signature about twice as large as $p$, when RSA signature (of the textbook variety of the reference) adds a signature about as large as $n$. It is clearly assumed that the public prime modulus $p$ is about the size of the RSA public composite moduli $n$.It follows that the size of the signature in the proposed scheme is twice that in RSA.