Nope, not lifesize. The silverfish is a shy creatue when in nymph form, but in its final adult stage which lasts upto 20 years, it can reach a size of 2 feet across the thorax.
At this stage in their life they have been known to show human like behaviour, and often awake home owners as they watch loud repeats of 'Question of Sport' in the early hours. And if that's not enough, shy as they still are, they will often empty your fridge of dairy produce (they love cheese), and hide in wardrobes/bathroom cabinets.