Q:

How to accurately measure brake calipers?

So I recently bought brake calipers only to realize they didn't reach my rims. In fact they only go to about the midpoint of my tire.
The calipers are similar to these :

Did I just happen to get a weird size? Are there standard sizes for different types of bikes?
What would be the best way to figure out what size I actually need?
(All of this is assuming you don't have the old brakes to measure)

A:

Caliper brakes have a reach measurement usually expressed as a range, representing the pad's ability to move in its slot. There are essentially 3 different reach classes for dual pivot brakes, but manufacturers vary in the exact numbers. For example most Tektro brakes have 39-51mm for short reach brakes, 47-57 for medium, and 55-73 for long. Shimano short reach brakes are 39-49, Campy is 40-50, etc. That number is the distance from the center of the brake mounting bolt to the center of the brake pad fixing bolt with the brake in actuated position. (I think the exact position it's measured in is with the faces of the arms that contact the back of the pads parallel, but I don't have proof of this.) To measure what reach a bike needs, measure from the mounting bolt hole to the center of the rim. 
There are longer reach classes within BMX/kids bikes and other bikes that use fat tires and caliper brakes. Usually these are the single pivot or u-brake/centerpull variety, although there are some uncommonly seen ones out there that are dual pivot sidepulls.
The vast majority of road bikes for the last 25-30ish years use short reach brakes. They're essentially the default for road bikes. This has led to a situation where many people are unfamiliar with anything else and use incorrect and confusing language, calling short reach brakes "standard" and brakes in the 47-57mm-ish group like you probably need "long". 
The 47-57 class was common on older road bikes and is used sometimes (never as much as it should be) to make caliper brake road bikes that can take wider tires and/or fenders. That's mostly over now though because disc brakes have more or less become the norm in that genre.

