23's are easier for glutes with me because it's hard to apply enough pressure when reaching back. 25's everywhere else though. I always heat under water first.
Higher G's (smaller pins) are tough for grainy substances, such as winstrol or Trenblone (fina from pellets). You should be fine with a 25G for test... it may be a slower injection but it won't clog.
Length... 1" for delts/quads and 1.5 for glutes. Have used some 1.5 on delts and quads but just don't stick it in that far.