change Font Size & Color of Rich Edit during runtime

You can use CFontDialog for font options as user click the button, here is the full code:
CFontDialog fntdlg;
if(fntdlg.DoModal()==IDOK)
{
TEXTMETRIC tm;
CClientDC dc(this);
GetTextMetrics(dc.m_hDC,&tm);
CHARFORMAT cf;
cf.cbSize=sizeof(CHARFORMAT);
cf.dwMask=CFM_BOLD|CFM_COLOR|CFM_FACE|CFM_ITALIC|CFM_SIZE|CFM_STRIKEOUT|CFM_UNDERLINE;
cf.dwEffects=0;
if(fntdlg.IsBold())cf.dwEffects=cf.dwEffects|CFE_BOLD;
if(fntdlg.IsItalic())cf.dwEffects=cf.dwEffects|CFE_ITALIC;
if(fntdlg.IsUnderline())cf.dwEffects=cf.dwEffects|CFE_UNDERLINE;
if(fntdlg.IsStrikeOut())cf.dwEffects=cf.dwEffects|CFE_STRIKEOUT;
cf.yHeight=tm.tmHeight*fntdlg.GetSize()/8;
cf.crTextColor=fntdlg.GetColor();
cf.bPitchAndFamily=fntdlg.m_cf.lpLogFont->lfPitchAndFamily;
cf.szFaceName[LF_FACESIZE]=fntdlg.m_cf.lpLogFont->lfFaceName[LF_FACESIZE];
SetCharFormat(cf);
}

I have used this code in one of my projects and it worked.
Ahmed Kamal

Note for both of ya:  instead of typing all that code, note that there is an undocumented CFontDialog constructor that takes a CHARFORMAT structure as its first parameter.  It is prototyped like this:

CFontDialog( const CHARFORMAT& charformat, DWORD dwFlags = CF_SCREENFONTS, CDC* pdcPrinter = NULL, CWnd* pParentWnd = NULL );

   Then use the undocumented CFontDialog::GetCharFormat(...) to get the user selected CHARFORMAT.

The quickest way to do it is to the following steps (Error checking left out):

   GetSelectionCharFormat( m_cfFormat );
   pFontDlg = new CFontDialog( m_cfFormat );
   if( pFontDlg -> DoModal() == IDOK )
   {
      pFontDlg -> GetCharFormat( m_cfFormat );
      SetSelectionCharFormat( m_cfFormat );
   }

   This will NOT work with RichEdit2.0 (CHARFORMAT2 structure).

-=- James.

Almost forgot...  If you want to have pre-set CHARFORMAT structures to apply formats with, create a little app that uses the CFontDialog stuff above, and when you get the CHARFORMAT that you want, store it someplace (file, registry, etc), and then you can use them in your application.  No need for the user to select something.

-=- James.

I want to "visual" change the char in the edit box. When the user click the button, the user can see the change at the same time.

You haven't specified your request clearly. I cannot answer a question whose owner isn't making it enough clear. May be next time I will try to help you, but this time, sorry.
Ahmed