Carbon exists in the forms of diamond and graphite which can pretty much be arbitrarily large (look up "number of carbon atoms in diamond"). It also exists in 60 carbon buckminsterfullerenes.
The largest homo-atomic molecule would be a diamond. Even a small diamond would be a single crystal with all the carbon atoms covalently bonded in a tetrahedral structure. That is why it is the hardest substance known.
Incidently, water molecules in ice crystals form a similar structure. That is why ice is so strong, like the iceberg that sank the titanic.