Help understanding lattice-based aggregate signature scheme
"Given parameters $n$ and $PP$, the KGC provides public/secret key pairs $(pk; sk)$ for all single signers."
  


Yes, the key generation center (KGC) generate the key pairs $(pk; sk)$ and distributes them. It is also clear from figure 3 of the article.


  
  What are the sizes of public keys, signatures, and aggregated signatures (in bytes)?
  


The parameters are used in $TrapGen(n,q,m)$, which actually generates for SIS parameters. You can find example secure paremeters from here or similar work.