Depending on the jurisdiction and your intent (based on the judgement of the responding authorities) entering any building could result in a felony charge.
If it is truly an unused commercial site, I doubt you would get more than a slap on the wrist trespassing charge, if not just told to leave. That's if you don't destroy anything or steal anything. The problem with big unused buildings is there is a lot of ground to cover and you could be blamed for damage done by someone else (vandalism and shit).
Go scope it out.
