Measuring Tack?

I am looking for some new tack but am not sure how to measure the current sizes I have.

For stirrup leathers, are they measured from buckle to the very end, buckle to the first hole, etc.?

For girths, are they measured just buckle to buckle?

For bit, is it ring to ring when laying flat, or from the VERY end of the mouthpiece, or from the outside of the ring, (this is a loose ring snaffle)?

Stirrup leathers are from end of buckle to end of leather
Girths are from end of buckle to end of buckle (so the ENTIRE length)
Bit is the length of the mouthpiece that actually lays in the horse's mouth. So for the loose-ring snaffle, you're measuring the length of the jointed mouthpiece from the inside of the one ring to the inside of the other ring.

Just a quick note on freia's otherwise perfect answer.

With loose ring bits, you should really measure from the edge of the hole the ring passes through in the mouthpiece to the same the other side. The problem with a lot of mass-produced bits is they measure from ring to ring so you may end up with a mouthpiece slightly shorter than you really want. I must say this is less important now flat loose rings have gone out of fashion as they were more likely to pinch a horse's lips where they passed through the mouthpiece.

One other small thing to bear in mind is a jointed or mullen mouth bit should be 1/2" longer than the corresponding straight bar mouthpiece. To allow for the curve as it sits in the horse's mouth.

Unclearthur, so a french-ling eggbutt should also be 1/2" longer than the actual "mouth-width" measurement? I knew to do this with loose-rings to avoid pinching, but should I also do that for the eggbutts, D-rings, and full-cheeks that have jointed mouthpieces? Am I understanding your explanation correctly?