If you are concerned about the IV there are things that help numb the site; L.E.T., EMLA cream or Ethyl Chloride (spray). See if the facility has a policy to use any of these. Also suggest to the nurse where they generally get your IV (sometimes the nurse will listen or not).
A butterfly is only a style of needle and not size. I use a small butterfly to draw blood or a very large butterfly when I worked in dialysis. Also most butterflies are just needles and not an IV catheter.
Good luck with your decision.