Dielectric relaxation times were measured for 1,4-polyisoprenes (PI) of different molecular weight. From the data, the number of dynamically correlated segments, N-c, was calculated using an approximation to the dynamic susceptibility. N-c increases with approach to the glass transition in the usual fashion and also increases with increasing molecular weight of the PI. The latter effect is ascribed to the loss of the configurational mobility conferred by the chain ends. The correlation volume was also estimated from calorimetry and, because PI has a dielectrically active normal mode, from the intersection of the extrapolated segmental and normal mode relaxation times. The three methods yield consistent results, although the last has large uncertainty due to the ambiguous connection between dynamic correlation lengths and volumes. Using the equation of state for the polymers, the dependence of the relaxation times on the scaling variable TV, where V is specific volume and gamma is a material constant, was calculated. For the lowest molecular weight PI, there is a small difference in gamma for the segmental and chain modes. The scaling exponent is also marginally smaller for the lower molecular weight sample, suggesting, in contrast with the behavior of other polymers, that in PI the volume dependence becomes weaker with decreasing molecular weight.