Schnorr signature security level as compared to AES/RSA As per the Schnorr's original paper (1991), 


  The Security Complexity $2^t$: We wish to choose the parameters $p$, $q$ so that forging a signature or an authentication requires about $2^t$ steps by known methods. For this we choose $q &gt; 2^{2t}$ and $p$ such that $2^t$ is about $e^{\sqrt{\ln(p)\ln(\ln(p))}}$. ... For $p &gt; 2^{512}$ and $q &gt; 2^{140}$ the discrete logarithm problem requires at least $2^{72}$ steps by known algorithms.


Now I have three questions regarding this.


How does value of $p$ comes to $2^{512}$ for security complexity of $2^{72}$?
What is the equivalent AES and RSA key-lengths if we use $p$ of $1024$ bits and $q$ of $512$ bits? 
Since Schnorr is finite field signature scheme, what is the 'field size', 'group size' and 'subgroup size' in case of schnorr signature scheme?


Thanks in advance.
How does value of $p$ comes to $2^{512}$ for security complexity of
  $2^{72}$?


Personally, I couldn't connfirm this number, but rather landed at $2^{66}$. If you don't know how to find this number, just plug the $2^{512}$ into the complexity equation given and take the logarithm to the base two for better readability.

Note that Schnorr's complexity estimate for the field is based on the Index-Calculus Algorithm and doesn't yet take into account the Number Field Sieve (original paper as PDF) against discrete logarithms, which would give a different expression, namely $L_p[1/3,3^{2/3}]=e^{(3^{2/3}+o(1))(\ln p)^{1/3}(\ln\ln p)^{2/3}}$, that would yield a much lower required attack complexity.


  What is the equivalent AES and RSA key-lengths if we use $p$ of $1024$
  bits and $q$ of $512$ bits?


$q$ is the subgroup order and if you get a 512-bit subgroup you'll have roughly 256-bit security for this subgroup. $p$ is the field prime and with it being 1024-bit, you'll get roughly 80-bit security against NFS-based attacks. So the answer is: A 1024-bit RSA modulus will have a comparable security along with AES-128, where 48 key bits are constant zero'ed out. Note that just plugging-in $2^{1024}$ into the complexity equations is a dangerous business, as $o(1)$ hides an implementation-dependenant (constant) factor that was not accounted for. Rather you need a concrete data point as a basis for further extrapolation. Here's a more detailed treatment of keylengths: (simple) (complex) by Lenstra and Verheul.


  Since Schnorr is finite field signature scheme, what is the 'field
  size', 'group size' and 'subgroup size' in case of schnorr
  signature scheme?


$p$ is the field prime and its bitlength is the field size.
$q$ is the subgroup order and its bitlength is the subgroup size.
There's no relevant "group size" in this case (maybe one could say that's equivalent to the field size though)