In all my HFSS simulations I always start with a cubic air box.
Set Box Parameters:
Position - (-x_box/2,-y_box/2,-z_box/2)
X = x_box
Y = y_box
Z = z_box
Make all dimensions of the box equal to your wavelength
If you haven't built up your model to be iso-centered on the origin of the axis, move it there.
Now you will have lambda/2 distance in all directions from the center of your structure.
This has always worked great for me. Don't forget you have to select the box and assign it a Radiation Boundary Condition...