  Most high-$T_c$ superconductors are spatially inhomogeneous. Usually, this
heterogeneity originates from the interplay of various types of electronic
ordering. It affects various superconducting properties, such as the transition
temperature, the magnetic upper critical field, the critical current, etc. In
this paper, we analyze the parameters of spatial phase segregation during the
first-order transition between superconductivity (SC) and a charge- or
spin-density wave state in quasi-one-dimensional metals with imperfect nesting,
typical of organic superconductors. An external pressure or another driving
parameter increases the transfer integrals in electron dispersion, which only
slightly affects SC but violates the Fermi surface nesting and suppresses the
density wave (DW). At a critical pressure $P_{c}$, the transition from a DW to
SC occurs. We estimate the characteristic size of superconducting islands
during this phase transition in organic metals in two ways. Using the
Ginzburg-Landau expansion, we analytically obtain a lower bound for the size of
SC domains. To estimate a more specific interval of the possible size of the
superconducting islands in (TMTSF)$_2$PF$_6$ samples, we perform numerical
calculations of the percolation probability via SC domains and compare the
results with experimental resistivity data. This helps to develop a consistent
microscopic description of SC spatial heterogeneity in various organic
superconductors.
