My grow area is of a similar size walk in closet, and my 400hps + 150 watt's of cfl's can get it quite warm with out ventalation the hps bulb gets hottt , it will get warm expecially a 1000 watt. You might want to invest in a cooltube. It also depends on the rooms tempature that the the ventalation is intaking the air from.

Here is a simple Exhaust Fan CFM Calculator
Hydroponics Exhaust Fan CFM Calculation for Gardening Indoors by Cheap Hydroponics

Hope this helps but the next project on my agenda for my 400 watt is a cooltube because there hotter then I though they would be lol.