You can use css
Padding for that.
.yourClass
{
padding-left:50px;
}
Live demo : CSS Padding
CSS Padding
UPDATE
<asp:Menu ID="Menu1" runat="server" DataSourceID="SiteMapDataSource1"
RenderingMode="List" StaticDisplayLevels="2" CssClass="MainMenu">
<LevelMenuItemStyles>
<asp:MenuItemStyle CssClass="SiteMapLevel1" />
<asp:MenuItemStyle CssClass="SiteMapLevel2" />
</LevelMenuItemStyles>
</asp:Menu>
CSS
.SiteMapLevel1
{
padding-bottom: 5px;
padding-top: 5px;
}
.SiteMapLevel2
{
padding-bottom: 5px;
padding-top: 5px;
}
Check here for more info : Using CSS and Styles with the Menu Control