Yes, science has measured and studied particles and molecules that are smaller than 5*10^-19. In fact, many subatomic particles, such as electrons, protons, and neutrons, have sizes much smaller than that scale.
For example, the size of an electron is estimated to be on the order of 10^-18 meters, which is smaller than 5*10^-19. Electrons are fundamental particles and are considered point-like, meaning they do not have a measurable size in the classical sense.
Additionally, experiments in particle physics, such as those conducted at particle accelerators like the Large Hadron Collider (LHC), have provided evidence for the existence of various subatomic particles with sizes smaller than the scale you mentioned.
As for molecules, their sizes can vary widely depending on their composition and structure. Some small molecules can have sizes on the order of a few angstroms (10^-10 meters), while larger and more complex molecules can have sizes spanning hundreds or thousands of nanometers. Scientists routinely study and manipulate molecules at these scales using various techniques, such as scanning probe microscopy and spectroscopy.
It's worth noting that the precise measurement and understanding of particles and molecules at such tiny scales are achieved through the framework of quantum mechanics, which is a branch of physics specifically developed to describe phenomena on the atomic and subatomic levels.