Constructs that gain class levels gain all the benefits, they just don't receive bonus hit points per level(that is: level x CON or level x CHA(for undead), they do get the roll of your hit die) - but rather based on size.
Advanced Race Guide has a 20 RP construct "player race", the Wyrwood p. 237 - pointwise they could be made medium at no extra cost.
I know I am resurrecting an old topic, and this may very well be a stupid question, but are the bonus hit points from size added PER HIT DICE, or simply a flat bonus of 10 at medium size at character creation?
I am assuming it is the later, but I just want some clarification.