There are 2 ways to do this.
1) add a style for this. and give cssClass for that label.For example,
.line
{
line-height:150%;
}
And add the class to your label.
<asp:Label ID="Label1" runat="server" cssClass="line" />
2) Add style in code behind
Label1.Style.Add("line-height","150%");
Change the line-height value to whatever you want