In terms of size, in the marine world best practice is </= 200ah with </= 100ah being ideal. As Sojourner states, mechanical failures have generally been cells of 400Ah and up. Up to you to decide how closely that use-case relates to yours.
In terms of casing type. Bearing in mind that there are usually exceptions to any generalized rule, the Nylon (plastic) cased cells, are generally regarded as more physically robust and durable than the majority of aluminum cased cells. Part of this has to do with the durability of the casing, part of this has to do with the build quality, and part of this has to do with the more robust terminals, which are supposedly actually integrated into the case structure, and beyond this are just beefier generally. The stud/bolt diameter and busbar quality are often superior as well. There is one aluminum cased cell that is considered fairly robust, that would be Frey (also known as Fortune, or Overkill Solar), there may be others but this is the one I am familiar with.
The Tradeoff with the more robust construction is somewhat less energy density (i.e. slightly larger and heavier cells for a given capacity).
For the sake of comparison here are some pictures of some of the popular Nylon Cased prismatic cells, as well as the Frey cells compared to the popular EVE aluminum cells (each image can and should be enlarged to view the terminals and build quality closeup):
CALB SE 100Ah | CALB CA 100Ah | Sinopoly 100Ah | Winston 100Ah | Frey 100Ah | EVE 100Ah | Eve 280Ah |
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And here is an example of a very robust installation with compression that is pretty easy to emulate. The only thing it is missing is a protective cover for the cell terminals:
I think either aluminum or nylon cased cells can be made secure, I think that in practice, aluminum cells take more care and require more external protection compared with their nylon cased cousins . Both have their pros and cons.