Use the Line Weights tab of the Modify DGN Export Setup dialog to map Revit line weights to MicroStation line weights.
Export
Options
(Export Setups DGN). Select the setup to modify. Click the Line Weights tab.
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.
To change all the values in the table back to their original settings, click Reset All.