I'm Enclosing My Furnace And Hot Water Heater In A 9x5 Room With 7' Ceiling
How Do I Now If The Combustion Air For That Room Will Be Okay

Your best guide would be the installation manuals for them. How much air you need will depend on the BTU input of both. I am not sure what all is in the decals on appliances. Check all yours carefully. There should at least be a plate giving the BTU's. If you give the brand and model, somebody may dig up what you need.

With both appliances in a room that size I can guarantee that there will not be enough combustion air. As labman said try to get information from the appliance manufacturer. If that fails just call the building department in your area. There code book addresses that issue.

On thing to remember is that 50% of the air has to be located high on the wall or ceiling and the other 50% low on the floor or wall area.

Legal tricks to consider.

Louvered doors that allow air to pass into the room. Then you are using the entire envelop of the house for combustion air.

Cutting a large non closeable register into the supply side of the heat duct/plenum in the small room. This trick will allow air to be drawn out of the supply duct when the furnace blower is not running and it will be blown out of duct when the furnace blower is on. This will allow a 50% reduction in square inch area of combustion air required but you will still need to bring in a smaller pipe from the attic or outside for the other 50%.

This trick has been used for years when I did inspections and saved people from having all that cold air dumping into there house.

Stoichiometric complete combustion requires 10Ft3 per 1000 btu/h of gas input. The National fuel gas code

Stoichiometric complete combustion requires 10Ft3 per 1000 btu/h of gas input. The National fuel gas code also recommends an additional 2.5Ft3 of excess air, hence you need: 12.5ft3 per 1000 btuh input.:)