If you start a fire in zero gravity then (as long as it is not against the wall of the space craft) it will burn as a sphere.  The fire needs oxygen so will expand, however, as it expands more heat is lost, and the fire may go out.\n\nSo it would be a hot, glowing sphere that expands and then goes out.\n\nOf course if it is contact with the wall or something then the nature of the fire depend on the fuels available, etc.
How do fire act in 'zero gravity'?