How big (or small) should the saddle bag be for a road bike needs?

I don't really want to carry my backpack everytime I go cycling, apart from commuting to work and back.

Ideally, I'd like to have at least one spare tube, a multitool, patches? And probably a tiny pump and tire levers. Not sure how the pump will fit in the saddlebag, but whatevs.

Generally speaking, it seems the most popular sizes to choose from is 0.6L, 1.0L and 1.3L.

I fear 0.6 will be too small, but then again I don't really want to overestimate my needs and end up with something riddiculously big.