When constructing walls around a gas furnace and water heater, how much room should I allow, ( how small can I go?) and how much ventilation does it need?

You are likely governed by a building code which can vary for locale so call the building department. We would do better if we knew more about the furance's efficiency rating and if it has a fresh air intake.

Okay, I asked about the building specs around a gas furnace and water heater, you needed more information. Here it is: The furnace is a Weil-Mclain Gold CGI it has a rating of 83.7 percent. It does not have a fresh intake vent only an exhaust. The water heater is a Top Performer manufactured by Vaughn MFG out of Salisbury Ma.

Do not start a new post. Put your answer back on the tail end of the original posts.