In the Star Trek Encyclopedia, 4th ed., vol. 1, p. 244, the Enterprise-J is identified as a Universe-class starship. The same reference book additionally described the ship as having an overall length of 3219 meters.
I can't find a reference for the Breen Dreadnaught size though, not enough beta canon material for something so new. I can't find a good source for Fed HQ size to compare either, just that it is actually a Pax class starship, but no size for those.