I recommend a size 22 needle. The complications and pain are much less with thinner needles and some even prefer to use a size 25 needle for arterial puncture. 

When the size 25 needle is used, gentle aspirations has to be done to withdraw blood. Inexperienced persons apply too much suction and trap air bubbles.
I prefer the spontaneous fill that occurs with the size 22 needle, as it minimizes the possibility of trapping air bubbles.

Larger sized needles are associated with more hematoma at the site of the puncture and are more painful.

REF: Sabin S, et al. Clinical experience using a small-gauge needle for arterial puncture. Chect 1976; 69:437-438.