Computer simulations were performed to examine the effects of the main (backbone) and side (branch) chain size on the shape and dimensions of a polymacromonomer using the bond fluctuation model in which bond cutting is allowed. The polymacromonomer was treated as a self-avoiding chain having N bonds in the main chain and n bonds in the side chains which are attached to every main chain element. In the simulation N and n were varied up to 64. A power law relation, 〈S2〉m∼N2νm, was obtained for each value of n, where 〈S2〉m is the mean-square radius of gyration of the main chain. The exponent 2νm increased monotonically from 1.24±0.02 at n=1 to 1.95±0.05 at n=64, indicating that the shape of the main chain gradually varied with increasing n, from a self-avoiding coil-like structure to an extended rod-like form. The mean-square radius of gyration of the side chain moiety, 〈S2〉s was independent both of N and also of the position of the branching point along the main chain for every value of n. A power law relation was also found between 〈S2〉s and n, with exponent 2νs=1.20±0.01. This exponent is the same as that for a linear chain, while 〈S2〉s is about 1.2 times the value 〈S2〉 of a linear chain with the same degree of polymerization. This strongly suggests that all side chains maintain a three-dimensional self-avoiding coil-like form, irrespective of the main chain, as it extends from a coil-like form to a rod-like form with increasing n.