For my tattoo removal, I went to Clinica Robles in Belgrano (http://www.clinicarobles.com/). I highly recommend them.
In this case, it is not the size of the tattoo only, per se, but rather by the zone and complexity. What I mean is that they explained to me that the older and darker in color a tattoo is, the easier it is to remove, because the ink is more "settled" in your skin, and if it is dark/black ink, it absorbs all colors of the laser that they use (I don't exactly understand the science behind it, but I guess it makes sense)...
To give you an example, I had an older tattoo of a phrase in black ink on my ankle, which started to disappear sooner than another with light blue, yellow, and pink on my wrist, which was my most recent one.
When I went for my consultation, since I ideally wanted to remove everything (a small wrist tattoo, and two tattoos that completely covered both ankles), after examining the tattoos, they also asked me if I had to "prioritize," which would be the one(s) that I would most like to get rid of. They then gave me an envelope with their prices (per session) for removing either all of them or just the few. They give you 72 hours to think it over and get back to them.
I never wound up finishing the process because I am pregnant, but they have ample office hours, the doctors are very professional, and their secretary is bilingual.
Just to let you know, it hurts if your tattoo is directly over a bone with no muscle (such as my ankles). My wrist was not as painful. Generally the first session hurts the worst, the others are relatively better. To help deal with the pain, they do offer anesthesia at no additional cost, but keep in mind that it is administered by various needle injections (injections directly over my ankle bones was NOT a pleasant experience!).
You have to have at least 30 days for the skin to heal between treatments. For the first 2 weeks or so, they prescribe you a specific ointment, and then once the skin has "repaired" itself more, they ask you to moisturize it daily with a non-scented lotion (like Nivea).
I hope this information helps! Also, a PSA- Just because you are 18 years old and are legally considered an "adult" does NOT mean that you know everything and it does NOT that you are capable of making brilliant decisions like tattoos! My only consolation was that at least I had the common sense not to get a tramp stamp!!!