  We report the comprehensive comparative analysis of the upper critical
magnetic fields $\mu_{0}H_{c2}(0)$ obtained within Ginzburg-Landau and
Werthamer-Helfand-Hohenberg theories for optimally-doped
$YBa_2Cu_3O_{7-\delta}$ thin films. For different orientations of the magnetic
field, our calculations give 638 and 153 T for $\mu_{0}H_{c2}(0)$, H || ab and
$\mu_{0}H_{c2}(0)$, H || c, respectively, when using $H_{c2}(0)$. For the first
time, the temperature dependences of coherence lengths $\xi_{ab}(T)$ and
$\xi_{c}(T)$ within proposed theories were determined using 50 and 90% criteria
of the normal state resistivity value $\rho_{N}$. The Ginzburg-Landau
0.9$\rho_{N}$ approach gives $\xi_{ab}(0)$=11.8 A, and $\xi_{c}(0)$=3.0 A which
are in a good agreement with literature data. The implications of very short
coherence lengths in HTSCs are discussed.
