I don't understand your question.
Armadillo is a math library, so it operates on vectors. If you do not know your size, you could allocate a guessed N
elements and resize in the common 'times two' idiom as needed, and shrink at the end. If you know the size, well then you have no problem.
The STL has the so-called generic containers and algorithms, but it does not do linear algebra. You need to figure out what you need most, and plan your implementation accordingly.