Size of DNA in phage
For lambda:


  If the distance between the two cos sites is less than ~37 kb, the resulting phage particle will be unstable. When the DNA is inside the capsid, it exerts pressure on the capsid. Likewise the capsid exerts an inward force on the DNA. If there is not enough DNA inside the capsid, it will implode from the inward force of the capsid. If the distance between the two cos sites is too far (~52 kb), then the capsid will be filled before the second cos is reached. The tail cannot be added because the DNA hanging out of the capsid is in the way and no infectious phage particle is produced.


http://www.microinmuno.qb.fcen.uba.ar/03-Bacteriophage.pdf

In contrast filamentous phage like M13 have no upper size limit, but since they get longer with more DNA they become physically fragile.