25g is always acceptable as long as you can push the oil through easily. The harder the struggle to push the oil through, the more pressure builds, kind of like kinking the garden hose and making the water shoot out harder. Imagine how the added pressure can cause issues/damage in the muscle. Length all depends on the muscle and how lean you are. I use 1inch for glutes, but I am fairly lean, under 8% and hardly can pinch any fat. For guys that have a layer of fat, you need to go with something longer. Heres an average :
glutes 1.5 in
Quads, lats and delts 1 in
Smaller muscles like Bi's, 5/8 can be acceptable for someone with lean arms.
Personally, I'd stick with the larger muscles as much as possible