Cherax peknyi (tiger/zebra variants), reaches 4-6" (male/female and variant depending). A very beautiful, but rather inactive crayfish. I have a female that's a little over 4" at the moment, still a young crayfish though.
"blue lobster" generally refers to Procambarus alleni or Cherax quadricarinatus. Procambarus alleni will usually max around 6", though there are reports of slightly larger individuals. Cherax quadricarinatus will usually max around 9", but if you happen to obtain an individual bred from a certain strain (like Walkamin) or that comes from a line of larger individuals, you can see them up to 14" and weighing about a 1lb (a bit over).
If you're looking for larger crays, definitely check out the Cherax quads, they're excellent beginner crays. Once you're experienced you may wish to look into other species like Cherax canaii and the like.
Also, since your post is states "blue lobster or a tiger lobster" are you looking for the "Blue/Black Tiger" crayfish? That species isn't very large, still an impressive crayfish, but it's illegal to own. Unless something has changed recently.
