I recently walled off my furnace room, and I need to know how much ventilation I would need for the room size of 9x25 feet. What size, what location, and how many vents would I need?
I do NOT want to install a louvered door.

For what purpose are you trying to install ventilaton - to condition the space or to provide combustion makeup air for proper furnace operation?

I was told an area around a furnace and water heater is supposed to have ventilation. I assume it's a safety issue.

If you were told that your furnace room needs ventilation - then I would suspect you need it for combustion air - that said what is the efficiency of your furnace? If it is low to medium efficiency then you have no outside combustion air and the furnace is using the inside house air for combustion and you should leave a gap at the bottom of the door 2" - this will allow the furnace to draw air from the house to allow it to combust. If you do not the furnace will have combustion issues with lack of air. If on the other hand you have a high efficiency furnace 90% or better then you will have a PVC pipe going outside bringing in combustion air and this post will be moot.

TG

You will nees to know the firing rate of the appliances in order to properly size ventilation openings (one near top of the room one near the bottom) Your local building official may be willing to assist. If not, why take a chance? Call a reputable heating and air conditioning contractor and hire them. You have a potentially dangerous condition there. Good Luck!