Touch range is 2.25 meters. This is meant to be an encompassing range that means giants could touch each other without constantly bumping into each other, or even a human casting Harm on a giant or dragon could do it without constantly walking into the model and not reaching the required distance. 2.25M is actually quite a long "touch" range for gnomes and halflings, and even humans look strange standing so far away to touch a target. It can bring the caster into attack of opportunity range, although sometimes it doesn't, if casting a hostile touch spell - for instance, Harm.