Small-angle neutron scattering (SANS) has been used to measure the chain dimensions of syndiotactic polypropylene (s-PP) in the melt, using mixtures of H-1- and D-2-labeled molecules. This leads to a segment length 7.6 Angstrom normalized to a four-carbon repeat unit. which is in excellent agreement with a prediction based on a correlation of chain conformation and the plateau modulus obtained from rheology. The s-PP segment length is substantially higher than the values of 6.2 Angstrom previously obtained for isotactic polypropylene (i-PP) in the melt and 5.6 Angstrom in a low-molecular-weight "polymeric solvent". We have also studied its effect on the thermodynamics of polyolefin blends by investigating the miscibility of s-PP with a range of atactic poly( ethylene/ethylethyelene) (PEEx) random copolymers. with x% ethylethylene, and comparing this with i-PP. The miscibility "window" of i-PP with PEEx ranges from x = 63-96% ethylethylene, while that of s-PP is shifted to x = 53-73%, in qualitative agreement with the concept that conformational symmetry matching favors miscibility. We have also performed a complementary measurement of the melt coil dimensions of s-PP, using a low-molecular-weight PEEx "polymeric solvent" similar to that previously employed for i-PP. A fit of the s-PP/PEE71 blend scattering yielded a segment length of 8.1 Angstrom, supporting the previous finding that the coil dimensions of s-PP are substantially higher than i-PP. These results emphasize the important influence of tacticity on the chain dimensions and thermodynamic properties in polyolefin systems.