Nevermind, I found it ! 
The values that I was looking for were stored inside the BaseLightmass.ini file (I suggest creating a custom DefaultLightmass.ini file inside your project).
[DevOptions.PrecomputedDynamicObjectLighting]
bVisualizeVolumeLightSamples=False
bVisualizeVolumeLightInterpolation=False
NumHemisphereSamplesScale=2
SurfaceLightSampleSpacing=75
FirstSurfaceSampleLayerHeight=16
SurfaceSampleLayerHeightSpacing=32
NumSurfaceSampleLayers=3
DetailVolumeSampleSpacing=25
VolumeLightSampleSpacing=1000
Those are my edited values, the variables names are mostly self-explanatory. FirstSurfaceSampleLayerHeight define the first sample height from the ground, then SurfaceSampleLayerHeightSpacing the incremented step size for the number of additional samples (defined by NumSurfaceSampleLayers).
For the horizontal spacing, you simply need to change SurfaceLightSampleSpacing.
