A 1% deviation in length would be more than adequate. The US AMA 2011-2012 competition rule cited below, though, does not say it.

"4.3: Weight and Size: No model may weigh more than 5000 grams gross, excluding fuel, ready for takeoff. Electric models are weighed with batteries. In Sportsman, Intermediate and Advanced an allowance of 115 grams is permitted. No model may have a wingspan or total length longer than two (2) meters (78.74 inches).
Wherever practical, all aircraft should be weighed prior to the start of the contest. If weight is being enforced then all planes competing in that class must be weighed before the same round (round 1 or otherwise) for a baseline. Only two attempts at making weight will be allowed on the day specified for weigh- in. If no calibration system is available for the scales, a tolerance of 50 grams will be allowed for possible inaccuracies in the measurement instrument."


I probably will cut the current spinner slightly to meet the 2m rule.