We use K−bandimagingtoinvestigatetheangularsizeandmorphologyof106Cradiogalaxies,atredshifts1\leq z\leq 1.4.Twoappeartobeundergoingmergers,anothercontainstwointensitypeaksalignedwiththeradiojets,whiletheothersevenappearconsistentwithbeingnormalellipticalsintheK−band.Intrinsichalf−lightradiiareestimatedfromtheareasofeachradiogalaxyimageaboveaseriesofthresholds.The6Cgalaxyradiiarefoundtobesignificantlysmallerthanthoseofthemoreradioluminous3CRgalaxiesatsimilarredshifts.ThiswouldindicatethatthehighermeanK−bandluminosityofthe3CRgalaxiesresultsfromadifferenceinthesizeofthehostgalaxies,andnotsolelyfromadifferenceinthepoweroftheactivenuclei.Thesize−luminosityrelationofthez\sim 1.16Cgalaxiesindicatesa1.0−−1.8magenhancementoftherest−frameR−bandsurfacebrightnessrelativetoeitherlocalellipticalsofthesamesizeorFRIIradiogalaxiesatz<0.2.The3CRgalaxiesatz\sim 1.1showacomparableenhancementinsurfacebrightness.Themeanradiusofthe6CgalaxiessuggeststhattheyevolveintoellipticalsofL\sim L^*luminosity,andisconsistentwiththeirlowredshiftcounterpartsbeingrelativelysmallFRIIgalaxies,afactor\sim 25lowerinradioluminosity,orsmallFRIgalaxiesafactorof\sim 1000$ lower in radio
luminosity. Hence the 6C radio galaxies may undergo at least as much optical
and radio evolution as the 3CR galaxies.Comment: 17 pages, 7 postscript figures, TEX, submitted to MNRA