We have always used the 23 gauge needles - 1" long.  They work great.  I know we would all like to use the smallest gauge possible, but be careful that the medicine will flow through easily.  I know we were supplied the wrong size needles with my daughter’s Lupron a few years ago (too small)  and it makes it very difficult to inject - and ended up more painfulfor her.  Remember the higher the gauge, the narrower the needle.

Are you flying?  We carried a letter from the doctor explaining why we were carrying needles and the airlines said we should be sure we had the exact same number needles/syringes as vials of medicine.

Good luck - and have a nice trip!