I would imagine the Seethra to be slightly bigger than man size - mostly due to their tough carapace. Maybe 2-3 metres tall? Their shells would have patterns and colours depending on the sub-species they belong to. The surrounding area / environment might also contribute to their coloration. While they do not have real colour-changing capabilities like squids, their patterns make them hard to detect if they stay still.
They can also bring their bodies close to the ground so that only the backs of their carapaces are visible. They often lie in wait for prey this way.
The sketch I made is that of a 2 legged crab-monster like thing with a pair of claws. I experimented with 4 legs and a pair of claws - that looks interesting too but I'll let you guys see the pic first and comment on it.
The Seethra, depending on species are quite diverse in their forms. Some are larger while some have heavier armour. One sub-species (Their name in the Seethra tongue translates to something like "duelists") has a rather exaggerated right claw. This claw is about twice to three times larger than the left and the left is smaller than one might expect of a Seethra of this size. Only the males seem to have this trait. They use it for, among other things, fighting and for display.
The Seethra, are in general territorial (even though they do not respect the territories of other creatures) and will fight to the death defending "their land". This leads to many feuds and often, all out war with the Merfolk who often live in the same environments. The fact that merfolk flesh is considered a delicacy amongst some of the Kulathra does not help the situation. (After all, how much can you befriend or respect something you can eat? This isn't Disney! :) )
The social structure of the Seethra vary from species to species. Some form somewhat large colonies while others are solitary. The larger ones tend towards the latter. Some form raiding bands or packs and prowl the seabed. These present dangers to underwater dwellers as well as surface dwellers who happen to swim in the wrong place. However, most Seethra have a patriarchal social structure.
This is quite obvious in the "duelists" subspecies. They form vast colonies under the sand. The dwelling themselves are mostly unconnected underground but they are in quite close proximity to each other. This creates opportunities for competition as well as bragging and display. The real advantage of such a colony, however is when an external enemy threatens the colony. The sight of an army (in the thousands) of marching duelists is an impressive one indeed.
For others, the patriarch is mostly consulted in matters of lore or legend - otherwise, they are left alone. Especially by the other males. These patriarchs tend to be large and wise (for a Seethra). They also tend to be the most dangerous if confronted in battle. After all, they didn't survive that long on good looks alone. There are tales of very large "crab monsters" 5 metres tall and 10 metres wide which some scholars say might be some very old Seethra patriarch. However, this is mostly dismissed by most surface dwellers as frightened or drunken sailors' tales than serious scholarly theories. Since no one has managed to capture any evidence of the monsters, these legends remain, for the most part, legends and stories to scare children.