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, < S-2>(m)similar to N-m(2 nu), was obtained for each value of n, where < S-2>(m) is the mean-square radius of gyration of the main chain. The exponent 2 nu(m) 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, < S-2>(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 < S-2>(s) and n, with exponent 2 nu(s)=1.20 +/- 0.01. This exponent is the same as that for a linear chain, while < S-2>(s) is about 1.2 times the value < S-2> 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.