BOSS shocks are of "twin tube" design. This is for several Reasons:
BOSS 12 way adjustable shock absorbers are at the forefront of shock absorber technology. 100% oil filled with a closed cell foam insert to ensure continued high performance under harsh conditions. Why 100% oil filled as opposed to gas/oil? Well it all comes down to cost.
A shock is measured by its extended and collapsed lengths. It is measured from the centre of the loop mount or hte base of the stud mount.
Remove it from the vehicle and allow it to expand on its own or pull it to the fully extended position and take a measurement.
Compress the shock by hand or by using a strap and take a measurement.
With the expanded and collapsed lengths of your current shocks, you will be able to find a suitable shock for your custom application
You will need to measure from the centre of a loop/pin style mount to the other centre of a loop/pin style mount.
You will need to jack and secure your vehicle and allow the suspension to drop. Jack the vehicle up until the wheel is off the ground, then lower it so the tyre is just on the ground. Take a measurement with the suspension at this point. If your vehicle currently has shocks on it, you will need to take removed them before measuring. As the shock could limit the suspension travel
You will need to compress your suspension fully. To do this, you can drive up a ramp with one side of the vehicle until the suspension is fully compressed. Or put a jack under one wheel to compress it
Once you have determined the correct size for your shocks, find one that is as close to possible to your measurements. Keep in mind that you may not find a shock that fits your vehicle exactly and that if available, More travel( a shorted collapsed length and longer extended length ) is preferable