  We report tunnel diode resonator measurements of in-plane London penetration
depth, $\lambda(T)$, in optimally-doped single crystals of
Fe(Te$_{0.58}$Se$_{0.42}$) with $T_c\sim$ 14.8 K. Systematic measurements were
carried out for six samples with different size and surface roughness. The
power-law behavior, $\Delta \lambda(T) = A T^n$ was found for all samples with
the average exponent $n_{avg} = 2.3 \pm 0.1$ and the pre-factor $A_{avg} = 1.0
\pm 0.2$ nm/K$^{2.3}$. The average superfluid density is well described by the
self-consistent two-gap $\gamma$ model resulting in $\Delta_{_{I}}(0)/k_B T_c$
= 1.93 and $\Delta_{_{II}}(0)/k_B T_c$ = 0.9. By analyzing the data obtained on
samples of different size and deliberately introduced surface roughness, it is
concluded that the calibration procedure used to obtain $\lambda(T)$ from the
measured TDR frequency shift is quite robust and the uncertainty in sample
dimensions and the nature of surface roughness play only minor role. The
exponent $n$, directly related to the superconducting gap structure, remains
virtually unchanged. The calibration - dependent pre-factor $A$ shows some
variation, but stays within reasonable margin ruling out some recent
suggestions that surface conditions can significantly affect the results. Our
results confirm that precision global measurements provide the most objective
information regarding temperature - dependent London penetration depth.
