According to International Residential Code (IRC), you'll need to provide a 30" x 30" level working space for servicing and inspection of both appliances. You'll also have to follow the manufacturers installation instructions, for clearances from combustible materials and any other restrictions.

You'll also have to consider supplying the equipment with adequate combustion air, and supplying the air from an appropriate source.

Working Space

International Residential Code 2012

Chpater 13 General Mechanical System Requirements

Section M1305 Appliance Access

M1305.1 Appliance access for inspection service, repair and replacement. Appliances shall be accessible for inspection, service, repair and replacement without removing permanent construction, other appliances, or any other piping or ducts not connected to the appliance being inspected, serviced, repaired or replaced. A level working space at least 30 inches deep and 30 inches wide (762 mm by 762 mm) shall be provided in front of the control side to service an appliance. Installation of room heaters shall be permitted with at least an 18-inch (457 mm) working space. A platform shall not be required for room heaters.

Water Heater

International Residential Code 2012

Chapter 28 Water Heaters

Section P2801 General

P2801.2 Installation.
Water heaters shall be installed in accordance with this chapter and Chapters 20 and 24.

P2801.3 Location. Water heaters and storage tanks shall be installed in accordance with Section M1305 and shall be located and connected to provide access for observation, maintenance, servicing and replacement.

Chapter 20 Boilers and Water Heaters

Section M2005 Water Heaters

M2005.1 General
Water heaters shall be installed in accordance with the manufacturer’s instructions and the requirements of this code. Water heaters installed in an attic shall comply with the requirements of Section M1305.1.3. Gas-fired water heaters shall comply with the requirements in Chapter 24. Domestic electric water heaters shall comply with UL 174. Oiled-fired water heaters shall comply with UL 732. Thermal solar water heaters shall comply with Chapter 23 and UL 174. Solid-fuel-fired water heaters shall comply with UL 2523.

Furnace

International Residential Code 2012

Chapter 24 Fuel Gas

Section G2408 (305) Installation

G2408.5 (305.8) Clearances to combustible construction.
Heat-producing equipment and appliances shall be installed to maintain the required clearances to combustible construction as specified in the listing and manufacturer’s instructions. Such clearances shall be reduced only in accordance with Section G2409. Clearances to combustibles shall include such considerations as door swing, drawer pull, overhead projections or shelving and window swing. Devices, such as door stops or limits and closers, shall not be used to provide the required clearances.

G2409.4 (308.4) Central heating boilers and furnaces.
Clearance requirements for central-heating boilers and furnaces shall comply with Sections G2409.4.1 through G2409.4.5. The clearance to these appliances shall not interfere with combustion air; draft hood clearance and relief; and accessibility for servicing.

G2409.4.1 (308.4.1) Appliances clearances.
Central-heating furnaces and low-pressure boilers shall be installed with clearances in accordance with the manufacturer’s instructions

G2409.4.5 (308.4.3) Clearance for servicing appliances.
Front clearance shall be sufficient for servicing the burner and the furnace or boiler.

Even if the furnace is direct vent, meaning it requires no interior air supply, it is still wise to allow air to circulate in the furnace space with the space outside it. Otherwise the temperature can potentially run quite high possibly leading to premature furnace component failure, especially the electronics.

During damp weather, my grandmother would use the furnace room as a drying room for laundry. If your space is not at a premium, perhaps making the enclosure extra large would help create a naturally green drying solution.