I added some known ships like the Alliance Cruisers or the Reapers to demonstrate the massive size of Kalros.
The common Thresher Maw we see in Mass Effect 1 and 2 are around 90 m based on the Codex entries and the wiki wich stated: "They can grow in excess of 30 meters above the ground, with a body nearly twice that size beneath the surface."
For Kalros i simply scale up the basic Thresher Maw until it match to the Reaper Destroyer wich epic fight we see in Mass Effect 3. So Kalros must be around 3 km long. Thats a big worm, right?
I use mostly the Reference pictures from for the starship side views.
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The max I got was 1350m that was giving the benefit of the doubt that in order to come up and on top of the destroyer; she must have had at most 450m of her body above ground, which is about a regular thresher maw’s visible body length over being 2.5 x the height of the destroyer. Having twice that length below ground would have maxed her out, at being 1530m upon going as far as stretching her out to being 3x the size of the destroyer. 2980m would still be nearly 2 and a quarter times larger than the max minimal length, and just 10m less than a regular thresher maw’s full body length below being exactly 2x the length of the max possible length of 1530m. So 3km might have over shot it a bit. Even if scaling the size up by using the method of her being 15 times larger and going another 15 for width and length, after cutting it by the it 3 for the visible 1/3 and hidden 2/3 of her full body legth, it would have left her at an absolute maximum length of 2,250m still placing her size here as being larger by nearly 8x a regular thresher maw's full body length. My Respect to Kalros and all, but I would'nt place her as being too much over 1.5km in length at max.
Also, Javik confirms in the Datapad app that Thresher Maws existed during the Prothean Cycle, wich means that, if the title "Mother of All Thresher Maws" was indeed true, Kalros will be over 50.000 years old.
PD: I always believe that Kalros is a 1000 meters long.