Common brake arm length on a road bike?
Reach is the distance between the center of the brake bolt and the middle of the brake shoe. Generally, brakes are sold with a range of reach (usually written on the brake; this is because you have some adjustment with the pads), like 40 mm - 50 mm or whatever and are termed as short, normal, long and extra long reach (but the actual reach lengths vary based on the manufacturer and time period of manufacturing, so you need to look at the range of reach for the particular brake you're using; for example, a long reach brake today may have been a normal reach brake in the 1960s). People also use drop bolts to use shorter reach brakes when longer reach brakes are required. 

Most brakes are probably 40-50 mm ish reach these days, though you need to check the particular brakes you're working with. 

To measure the reach needed, take the wheel (ideally, without a tire), and remove the existing brake. Then, use a ruler to measure from the center of the brake hole to the center of where the brake should hit the outside of the rim. If this is 45 mm, pick a brake such that the reach range contains 45 mm (e.g. a 40-50 mm reach brake).