On the Line Weights tab of the Modify DGN Export Setup dialog (see Export to DGN), you can map Revit line weights to specific DGN line weights as needed.
To change the default setting for a Revit line weight, click the corresponding value in the Line Weights in DGN column, and select the desired value from the drop-down list.
You can click Reset All to change all the values in the Line Weights in DGN column back to their original setting.