The image quality performed by ground-based large telescopes is frequently better than envisioned from DIMM-measured seeing. This improvement is one of the effects of a finite outer scale (L0) on the wavefront of the light propagating through atmospheric turbulence. Moreover, finite L0 also impacts the wavelength dependence of the full width at half maximum (FWHM) of point-like sources in long-exposure observations, deviating from the natural wavelength dependence of the seeing (lambda^-1/5). Seeing-limited integral field spectroscopic (IFS) data allows checking the FWHM wavelength variation for long-exposure observations by recovering narrow-band filter images of point-like sources. These images are observed homogeneously under the same instrumental and atmospheric conditions. Deviations of such FHWM(lambda) from the natural seeing provide an approach to the L0 during the IFS observation. We present the analysis of FWHM with lambda for point-like sources obtained from the Multi Unit Spectroscopic Explorer (MUSE) in the Wide Field Mode to approach the atmospheric outer scale at the Paranal observatory.