We develop a first-principles framework for evaluating the fundamental length scales of superconductivity, namely the coherence length ${\xi }_0$ and the magnetic penetration depth ${\lambda }_L$, within superconducting density functional theory (SCDFT). By incorporating finite-momentum Cooper pairs, we formulate a microscopic scheme that enables a consistent and parameter-free determination of ${\xi }_0$, ${\lambda }_L$, and the superconducting transition temperature $T_c$ on the same theoretical footing. Applying the method to representative elemental superconductors, the A15 compound V${}_3$Si, and H${}_3$S under high pressure, we obtain results in good agreement with available experimental and reproduce the type-I/type-II classification across all materials studied. The unified access to ${\xi }_0$ and ${\lambda }_L$ further allows us to construct the Uemura plot entirely from first principles, showing that higher-$T_c$ systems are characterized by the simultaneous realization of strong pairing and large phase stiffness. Our results establish a predictive first-principles route to superconducting length scales and provide a microscopic interpretation of empirical correlations in superconductivity.