int numCols = ((DataTable)(dataGrid1.DataSource)).Columns.Count; // кол-во колонок
int scrollBarWidth = dataGrid1.VerticalScrollBarVisible ? SystemInformation.VerticalScrollBarWidth :  0 ; // ширина scrollbara

int runningWidthUsed = this.dataGrid1.TableStyles[ "customers" ].RowHeaderWidth; 
for(int i =  0 ; i < numCols; i++)
	runningWidthUsed += this.dataGrid1.TableStyles[ "customers" ].GridColumnStyles[i].Width;
dataGrid1.Width = runningWidthUsed + scrollBarWidth +  4 ;