Are Large Blisters Common After Laser Therapy?

I would first comment that it is important to know what laser and for what condition your were treated. With that said, blisters after laser therapy are not a normal, expected or desired result (though mild blistering/crusting may be an expected temporary outcome a few days after some laser tattoo treatments).
The goal of most cosmetic laser therapy is to selectively target the source of your concern, while protecting the surrounding skin and tissues. Blistering is the result of unwanted trauma to the surrounding skin and can be caused by a myriad of factors--- improper laser selection for the condition being treated, too much energy employed, improper skin cooling, poor laser operator technique, laser technical failure, treating tanned or sunburned skin-- the list goes on.
This is why we continually stress the need to be evaluated and treated by a physician trained in a accepted field of aesthetic medicine (such as Dermatology or Plastic Surgery), whom you trust, and who has extensive experience with laser theory, safety, and treatment. The physician should be personally treating or directly supervising (on site) your treatments. Finally, you will find that, in general, more experienced physicians will own (not rent) multiple lasers.

The legs heal very slowly.  That is a long time to be healing, though.  If you had a ruby laser, I would expect a week or so of your legs looking bad.  But for most lasers, you should be healed. I would have to know your  laser and therapy for an accurate answer.  I would also not rule out that you are having a reaction to the Neosporin and inflamed from the medication.  I would switch to Aquaphor.  Be sure to talk with your doctor about this first.

If appropriate precaution is taken (e.g. perform test area, icing if chilled tip not availabl) , there should not be any blistering or scabbing after laser hair removal. When blistering or scabbing occurs, you should demand to be evaluated  by the supervising physician, start lubricating the open sores and possibly start oral antibiotics to prevent secondary infection. This is illustrative of the need to ascertain the supervising physician of a laser clinic be a board-certified dermatologist or plastic surgeon who is competent to manage post laser complications such as blistering and/or post-treatment pigmentation.