I use the 25 gauge, 5/8" pins too. I've also used 1 inch but that's a bit deep. My arms are only 16-17".
I think its better to shoot into the glutes if they are available. The termporary extra size you get in the arms is kinda fun but a little uncomfortable sometimes. I do it not in the hope of getting extra site growth, but just because when using trenbolone I need every site I can get.
I've put up to 3 ccs but that's really too much for my arms. I do no more than 2 ccs, 1 1/2 is better. I only do arms once a week, alternating bis one week and tris the next.
I shoot all my gear through a 25g pin...takes a lil longer, but its worth it IMO just dont try to drain with a 25g cuz its slower than the second coming of christ and if you try and shoot with that same needle it doesnt poke through the skin very easily