Amateur comes from the word amore which means love. Amateur or home haunts do it for the love of it. Professional haunts do it for money, sure many of them love it too, but they rely on the haunted attraction for a portion (if not all) of their income. Hopefully they also love what they do.
A pro haunt is one that charges admission and a the profits (note the word profits) go to the haunted attraction owner. A charity haunt also charges admission but its profits go to a charity.
That is technically the only requirement. Hundreds of haunts are pro haunts that are not involved in the haunting community in any way whatsoever. They are good haunts, but lack the diversity of haunts with creative staff that do get to see other shows and other operation systems. I call these "gold fish bowl" haunts. They are big fish where they are and think they are masters at what they do. The other end of the spectrum are folks who have little to no haunt experience and are deeply involved in the daily haunt chatter and talk on message boards and at trade shows. They know everything and come up with "new" ideas and they do not have the foggiest idea as to why the ideas are doomed. I call them "soft shells" because they are like molting crabs they have some knowledge of the world but the armor of experience has not yet hardened so even terrible ideas get right through it. They have potential to be good haunters if they stick with it and learn the business. We just have to peel them off of the internet and the 400 different web pages about "corpsifying a skeleton" so they can make a dozen or two for themselves.
Perhaps I ramble, now to answer your questions.
Is it based on size?
No if you pay $3 to go into a haunted closet and an actor yell boo while your there, you just paid to go through a professional haunt.
How long you have been in business?
How much money you generate or loose?
Nope you can lose money and still be a pro....heck you can lose money every year and as long as something keeps you afloat you still have a prohaunt. (more shows are in this pattern than you realize).
How many patrons attend your attraction?
Nope, business sucks at alot of pro haunts.
Do you need more then one attraction?
Nope, one show no matter the size will do ya.
If you are part of an association?
The associations do not legitimize you, lots of great people and lots of idiots belong to associations. They are a great way to get involved in the national haunt community but they will not affect your daily life or your season unless you choose for them to. they are a great resource...but then again if you can stomach all the BS, flame wars, and sales pitches this board is a far more valuable tool than any association I have or have not joined.
A Pro haunter makes money (in theory) from his show. However in my opinion (Im always short on those aren't I?) A good haunter is one who is willing to share their knowledge on set design, costuming, actor training, and operations with others for the betterment of the industry.