Sonar is man’s only effective tool for conducting an underwater search. It relies on pulses of sound which reflect from objects in the water providing a return signal which can then be interpreted.
In particular, sonar is able to detect objects which have different densities to that of water. The loch walls reflect strongly as they are made of stone, while the silty loch bed gives a weaker trace.
Air reflects very strongly whereas flesh is almost the same density as water so reflects weakly. When searching for animals with sonar, it is the lungs in a mammal or reptile, or the swim-bladder in a fish which gives the bulk of the reflection.
It is not possible to give a size of an object from sonar, only a strength. You might imagine the two are the same, but that is not the case. For instance a man swimming through the beam with an aqualung would appear very strong, but it is the aqualung and the air trapped in his mask and suit which gives the bulk of the reflection.
When the 1987 Operation Deepscan obtained three very strong contacts, it was not possible to give a size because different animals have different contact strengths. An eel, for instance, has no air sac in its body so would have to be enormous to register at all.
Experienced users of sonar can estimate the size of objects from past experience and Darrell Lowrance of Operation Deepscan estimated that their strongest contact was larger than a shark, but smaller than a whale.
I’m sure most of us would be happy if Nessie were within that range.