This uses partitioning, with padding if required, to make sublists.
f = Module[{b, c = 1},
While[Length[b = Union@Partition[#, c, c, {1, 1}, Take[#, c]]] > 1, c++];
{Length@First@b, First@b}] &;
Example
f@{73, 7, 4, 73, 7, 4, 73, 7, 4, 73, 7, 4, 73, 7}
{3, {73, 7, 4}}