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They usually are square, but they can be rectangular, too. Important is that they are unique (all the same size). Contacts and vias are the most numerous data structures in a layout, so their OPC data prep CPU time has to be minimized, hence this uniqueness requirement, s. e.g. this thread.
It's about process control. You can tune an etch process for
one feature geometry much better than you can when it has
to cover a range - especially when the most-used is at the
barely-capable end of the tool set, you may need to favor it
to exclusion of other sizes (which otherwise would tend to
"blow out" larger than as-drawn, and any contact-defined
device attributes possibly be uncontrolled, or min-contact-
consistent overlap rules violated, or whatever).
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