B777 DEP FROM IAD DESCRIBED GAR EVENT WHEN CTLR ERRED IN SPACING JUDGEMENT ISSUING DEP CLRNC AND THEN RESCINDING CLRNC.

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TWR CLRED US INTO 'POS AND HOLD' ON RWY 1R; AFTER A PRECEDING RJ HAD TAKEN OFF. AS WE WERE PASSING THE 'HOLD SHORT' LINE; TWR ASKED US TO HOLD POS; AND THEN CLRED ACR Y (ALSO A WDB) TO LAND ON RWY 1R. ESTIMATE ACR Y AS APPROX 2.5 NM FROM TOUCHDOWN WHEN CLRNC TO LAND WAS ISSUED. I IMMEDIATELY CALLED TWR AND ADVISED THAT OUR ACFT; PURSUANT TO TWR CLRNC; HAD ALREADY ENCROACHED ON THE RWY PAST THE HOLD SHORT BY ABOUT 20 FT. TWR ASKED IF WE COULD EXPEDITIOUSLY TAXI ONTO RWY 1R; THEN TAXI CLR ONTO TXWY K8 TO ENABLE THE APCHING ACR Y ACFT TO LAND. I DECLINED; AS IN MY OPINION; IT WOULD HAVE BEEN TOO CLOSE BY THE TIME WE USED (EXCESSIVE) THRUST TO GET MOVING AGAIN; POSSIBLY THREATENING ANY SMALLER ACFT BEHIND US WITH JET BLAST DAMAGE; TURNED ONTO THE RWY; TAXIED THE SEVERAL HUNDRED FT TO ENABLE US TO BE IN A POS TO MAKE THE L TURNOFF ONTO K8; THEN TO MAKE THE TURN AND ACTUALLY CLR THE RWY. TWR THEN RESCINDED ACR Y'S LNDG CLRNC AND ISSUED GAR INSTRUCTIONS. THE CTLR WHO ISSUED THE CONFLICTING INSTRUCTIONS TO US WAS UNABLE TO SEE WHETHER WE WERE BEYOND THE HOLD SHORT LINE; AND HENCE ISSUED THE LNDG CLRNC TO ACR Y. CONCERNS: 1) CTLR SHOULD RESPECT THE DIFFERING SIZES AND RELATIVE MOBILITY OF DIFFERENT SIZES OF ACFT. IT IS CONSIDERABLY MORE DIFFICULT TO MANEUVER A B777 THAN AN RJ. 2) CTLR SHOULD NOT HAVE ISSUED LNDG CLRNC TO ACR Y AFTER WE HAD BEEN CLRED INTO POS AND HOLD. 3) THE ERROR WAS COMPOUNDED BY THE SUBSEQUENT REQUEST TO PUT MORE METAL INTO POSSIBLE CONFLICT WITH THE APCHING ACR Y HVY JET BY ASKING US TO TAKE THE RWY AND 'HURRY UP' TO CLR IT ON K8 IN ORDER TO REMEDY THE IMPENDING TFC CONFLICT.