mi sin tawa toki pona
...but in answer to your question, surely it would depend on the context?
So, assuming I'm getting the hang of the lingo...
For 'square' in the pure geometric sense I think I would use 'sitelen (pi) linja (sama) tu tu'. I also considered 'kulupu (pi) linja (sama) tu tu', but I think 'sitelen' is better in most contexts. 'sitelen (pi) poka (sama) tu tu' might be even better now that I think of it. I think 'pi' is unnecessary in all of these cases, but I'm at the edge of my confidence in my competence here.
On the other hand, 'town square' could be something like 'ma jan ale pi ma tomo' or 'ma insa pi ma tomo' (perhaps with 'supa' thrown in there somewhere?). Could you have used 'leko' in that context, i.e. 'leko pi ma tomo'?
A woodworking square could be 'ilo linja pona pi kiwen kasi'. I can't think of a better alternative (taso la mi sin).
A square table could be 'supa pi poka sama' (I'm not sure if I'm using 'pi' right there, but I think 'supa poka sama' would be 'side-table of sameness' or perhaps 'the same side-table' or more likely 'side-table like ...." begging a noun phrase.)
'Stair' (singular) is a bit odd, but 'stairs' could be 'nasin lawa' or 'nasin anpa' in most contexts, I think. So 'stair' perhaps 'nasin lawa/anpa wan'? 'supa' might fit in there somewhere too. Or 'supa noka (wan) pi nasin lawa/anpa' perhaps?
Block could be as simple as 'kiwen', but more likely with situational modifiers, e.g. 'kiwen pi tawa noka wawa' perhaps, for a sprinter's block?
ona li pona ala pona?