I had explained myself wrongly, sorry.
I increased the surface area of the device, extending it 1 meter on each side from the burner, just in case. I think it doesn't change for the result.
I created a mesh with 0.10m x 0.10m x 0.10m cells.
With these arrangements, FDS returns me a mass flow rate of about 16 kg/s, which is very close to the calculated value (13 kg/s).
I also tried to calculate the average visible flame height. The analytically calculated value is about 0.53m, FDS returns me a height of 0.60m.
I think I could consider my model to be sufficiently correct.
Now, I need to provide a smoke disposal system, which will evacuate 13 kg/s of smoke. Considering hot gas temperature 360 K and hot gas density 0.98 kg/m^3 I derive a flow rate of 48233 m^3/h.