5000 BTU's should work for a room that size.
It depends on room size, including ceiling height. In a 9x12 foot room with 9-foot ceilings, my 5000 BTU air conditioner cools to about 60-degrees.
usually around 400sq. ft room give or take
5000 BTU is a tiny amount of cooling - are you sure you don't mean 50,000 BTU? A 5000 BTU might give you a drop of 5 degrees C in a 6' x 8' office with no south facing windows, as long as the ceiling isn't too high. For a normal 10' square room, 12,000 BTU is a more reasonable starting point, then add more for sources of heat gain in the room.
If measuring heat, a BTU measures the amount of heat that is required to raise the temperature of a pound of water to 1º Fahrenheit. In terms of air conditioning, the BTU determines the amount of heat the unit can remove from the room. As the BTU rating increases, so does the size, weight and cost of the unit.
Rule of thumb, around 300 sq ft give or take.
500sq.ft
There is a lot more to it than square footage. To find out the heat load on any room or home you need to have a load calculation performed. This is a process that takes into account location, construction features, etc. With a portable heater (which is what you're talking about if it's 5,000 BTU), you can estimate heating ability with the 10 - 15 watt per square foot rule of thumb. So, 1,500 watts (5,120 BTU) will heat about 150 - 225 square feet. To go from watts to BTU, multiply watts by 3.4.
It depends on your enviornment. A 36,000 BTU unit will probably work just fine. 36,000 btu's would heat a 1,000 square foot room. or more.
It takes about 50,000 btus to heat 1000 square feet
At least 8000 btu
1500