If you set 50 in the Cell Size Window in the Option Tab of the Spatial Analyst tool and carry out(for example) a Reclassification (it is non needed to complete the operation, it is only necessary to start it), then also the Tabulate Area tool will use 50 as processing cell size (regardless of the value set in the Processing Cell Size of the Tabulate area tool); if you at this point need to change again cell size, e.g. 250, the only way to set this value is to set it in the Option Tab of the Spatial Analyst tool and perform an operation (e.g reclassification) with this cell size (exactly as done before).