setting font size at run time

The size property is not read-only.

Here is the metadata for the class. Check this

namespace System.Web.UI.WebControls
{
// Summary:
// Encapsulates the font properties of text. This class cannot be inherited.
[TypeConverter(typeof(ExpandableObjectConverter))]
public sealed class FontInfo
{
// Summary:
// Gets or sets a value that indicates whether the font is bold.
//
// Returns:
// true if the font is bold; otherwise, false. The default value is false.
[WebSysDescription("FontInfo_Bold")]
[WebCategory("Appearance")]
[DefaultValue(false)]
[NotifyParentProperty(true)]
public bool Bold { get; set; }
//
// Summary:
// Gets or sets a value that indicates whether the font is italic.
//
// Returns:
// true if the font is italic; otherwise, false. The default value is false.
[NotifyParentProperty(true)]
[WebCategory("Appearance")]
[DefaultValue(false)]
[WebSysDescription("FontInfo_Italic")]
public bool Italic { get; set; }
//
// Summary:
// Gets or sets the primary font name.
//
// Returns:
// The primary font name. The default value is System.String.Empty, which indicates
// that this property is not set.
//
// Exceptions:
// System.ArgumentNullException:
// The specified font name is null.
[Editor("System.Drawing.Design.FontNameEditor, System.Drawing.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(UITypeEditor))]
[DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
[TypeConverter(typeof(FontConverter.FontNameConverter))]
[WebCategory("Appearance")]
[DefaultValue("")]
[WebSysDescription("FontInfo_Name")]
[NotifyParentProperty(true)]
[RefreshProperties(RefreshProperties.Repaint)]
public string Name { get; set; }
//
// Summary:
// Gets or sets an ordered array of font names.
//
// Returns:
// An ordered array of font names.
[TypeConverter(typeof(FontNamesConverter))]
[RefreshProperties(RefreshProperties.Repaint)]
[NotifyParentProperty(true)]
[WebSysDescription("FontInfo_Names")]
[WebCategory("Appearance")]
[Editor("System.Windows.Forms.Design.StringArrayEditor, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(UITypeEditor))]
public string[] Names { get; set; }
//
// Summary:
// Gets or sets a value that indicates whether the font is overlined.
//
// Returns:
// true if the font is overlined; otherwise, false. The default value is false.
[NotifyParentProperty(true)]
[WebCategory("Appearance")]
[DefaultValue(false)]
[WebSysDescription("FontInfo_Overline")]
public bool Overline { get; set; }
//
// Summary:
// Gets or sets the font size.
//
// Returns:
// A System.Web.UI.WebControls.FontUnit that represents the font size.
//
// Exceptions:
// System.ArgumentOutOfRangeException:
// The specified font size is negative.
[RefreshProperties(RefreshProperties.Repaint)]
[WebCategory("Appearance")]
[DefaultValue(typeof(FontUnit), "")]
[WebSysDescription("FontInfo_Size")]
[NotifyParentProperty(true)]
public FontUnit Size { get; set; }
//
// Summary:
// Gets or sets a value that indicates whether the font is strikethrough.
//
// Returns:
// true if the font is struck through; otherwise, false. The default value is
// false.
[NotifyParentProperty(true)]