A line support is added with the 'line support on 1D' command.
When changing for example Z to 'flexible', you'll notice that the stiffness is in [MN/m²].
If we enter a value of 1 MN/m², that would mean that in case of a uniform loading on the element we receive a reaction force of 1 MN/m for each m that the beam is displaced in the Z-direction.
So this means that a flexible line support on a 1D member does not take the width of the member into account automatically and you still might have to multiply your stiffness with the width manually.
If you want to take into account a width as well, you could choose for the Foundation strip type. In that case you'll define a 'thickness' as width and a subsoil with a spring value in MN/m³.