  The possibility of an unconventional form of high temperature
superconductivity in flat band (FB) material does not cease to challenge our
understanding of the physics in correlated systems. Recently, it has been
argued that the coherence length in FB compounds could be decomposed into a
conventional part of BCS type and a geometric contribution which characterises
the FB eigenstates, the quantum metric. Here, we propose to address this issue
in various FB systems and discuss whether the extracted characteristic
lengthscales such as the size of the Cooper pairs obey this conjecture. It is
found that the relevant lengthscales are less than one lattice spacing, weakly
sensitive to the strength of the electron-electron interaction, and more
importantly disconnected from the quantum metric.
