You may want to contact your county sheriff's office or local police department to see if they have a program to enforce no trespassing on your behalf. Here in Phoenix, business and land owners can sign a form with the city giving the police the authority to enforce no trespassing bans without you being present at the property. If they do have such a program, they can assist with proper signage, but as far as I know there isn't a requirement for size of sign, color, or placement. I think the signs just need to be conspicuously placed so that a trespasser can't act like he didn't see it.
Attorney David Kephart is an experienced Criminal Defense Trial Attorney and Jury Consultant. He is the recipient of the Arizona Attorneys for Criminal Justice President's Award and the recipient of the International Academy of Trial Lawyers Commendation for Excellence in Trial Advocacy. His response to your question is intended to be a general statement of law, should not be relied upon as legal advice, does not create an attorney/client relationship, and does not create a right to continuing email exchanges.