If you set the cell size environment to the result of valueAsText and the parameter is a raster path, the value set will be the raster's meanCellHeight (as string).
>>> from arcpy import env
>>> env.cellSize = "copy_ras"
>>> env.cellSize
u'10'
If the user does not select a dataset but instead leaves it at Maximum of Inputs, etc. will be a number or a string "MAXOF", "MINOF", etc.
arcpy.env.cellSize = cell_Size.valueAsText
try:
cell_Size = float(env.cellSize)
except:
arcpy.AddError("Cell size not numeric: {}".format(cell_Size))
You could do this test in the parameter validation if you wanted to trap it before the tool runs.
Another approach I have used is to define my cell size parameter as Double and used parameter validation to have it default to the environment cell size (if set).