The aim of this work is to study the effects of environment on the growth in size of galaxies. We examine the stellar mass-size relation for a sample of isolated galaxies interpreted as stellar systems where evolution has been mainly governed by internal processes. Effects of environment on the stellar mass-size relation are evaluated by comparing our results with samples of less isolated early- and late-type galaxies, as well as, for the first time, different spiral subtypes. We find no difference in the stellar mass-size relation for very isolated and less isolated early-type galaxies. We find that late-type isolated galaxies are ~1.2 times larger than less isolated objects with similar mass. Isolated galaxies and comparison samples were divided into 6 morphological ranges (E/S0, Spirals, Sb, Sbc, Sc, and Scd-Sdm) and 5 stellar mass bins between log(M*)=[9,11.5]. In all cases the relation is better defined and has less scatter for the isolated galaxies. We find that as the morphological type becomes later the galaxy size (for a fixed stellar mass) becomes larger. For the lowest stellar mass bins log(M*)=[9,10] we find good agreement between sizes of AMIGA and comparison spirals. The isolated spiral galaxies in the high stellar mass bins log(M*)=[10,11] tend to be larger than less isolated galaxies. This difference in size is found for all spiral subtypes and becomes larger when we compare fully isolated galaxies with galaxies having 2 or more satellites (neighbors within 3 magnitudes of difference at a distance less than 250 kpc from the galaxy). Our results suggest that massive spiral galaxies located in low density environments, both in terms of major companions and satellites, have larger sizes than samples of less isolated galaxies. Hence the environment has played a role in the growth in size of massive spiral galaxies.

11 pages, 5 figures, accepted for publication in MNRAS