You want to measure across, from the center of each hole (the "B" measurement). Note that some holes will be similar to a slot (this allows you more room to get the bolt holes to align when installing). The open "feet" ends also allow a bit more room for alignment. (the illustration gives you an example of both types).

In some cases, mechanics have been known to use a drill to change the shape of the holes or make them slightly bigger. BUT NOTE: These holes are very small, usually around 7/16's or less, so you will not have a lot of room to make them bigger.

On a stud / stem end, you want to measure the "A" measurement. From the bottom shank to the end of where the threads end. While not always critical, you may need to know how much room you have for the rubber (or urethane) bushings. For instance, if the stem is 2 inches long, but the bushings are each one inch thick, that leaves little room for placement and for the nut. So keep that in consideration. Most shock stems are a bit longer than they need to be anyway, but it's good to measure them and know the dimensions.

On a "loop" end, you need to determine the size of the bolt hole needed. (the shaded yellow in the diagram above indicating a bolt).

If you have loose bolts around the shop, you can try different sizes to determine this just by seeing what bolt fits in the old shock. Or you can measure the hole or old bolt with a tool, such as a Dial Caliper. Be aware on a OE vehicle, the bolts used could be Metric or Standard.

If there is no old shock absorber, and you are starting with nothing, then you need to see what size the existing shock mounts are on the vehicle / project. (or fabricate your shock mounts on the chassis accordingly).

If you haven't started yet, and have no mounts or shock absorbers, your best bet would be to use the largest bolt you can for strength if needed. Meaning, using a 5/8 or 3/4 bolt (and appropriate mount / size shocks).

Fabricate a mount to use the larger bolt. Bolts on cars and trucks are normally Grade 8 or better.

An exception to this would be if you are using a HEIM joint (spherical rod end). The racing standard for these is usually 1/2 inch on most shocks. (and you would have to use a 1/2 bolt or mount stud).

Loop end shocks are the easiest to work with on custom projects, and offer the greatest strength at each end.

They also can be easily modified. The bolt holes sometimes can be drilled out to accept larger bolts, or they can be sleeved to accept a smaller bolt. Bushings can also be pressed out and replaced with urethane ones of different sizes if needed.

Mechanics have also been known to "thin out" the mounting loop by cutting it as well, to make it fit into a U-shaped bracket mount on a chassis / frame / project.

This is usually done by using a cutting tool, such as a hacksaw, to shave a bit off each side of the loop.