We studied the size-surface brightness and the size-mass relations of a sample of 16 cluster elliptical galaxies in the mass range ~ 10sup 10 -2 10sup 11 M, which were morphologically selected in the cluster RDC S J0848+4453 at z = 1.27. Our aim is to assess whether they have completed their mass growth at their redshift or significant mass, and/or size growth can or must still take place until z = 0. We conclude that these 16 cluster ellipticals at z = 1.27 have, for the most part, completed their stellar mass growth at the redshift they are and that consequently, their evolution at z <1.27 will be dominated by the aging of their stellar content. This evolution would be instead mainly driven by disk galaxies. We do not find hints of differences between the properties of these cluster ellipticals and those of field ellipticals at comparable redshift, even if this last comparison is still based on a low number statistics.