private struct PARAFORMAT2
{
	public int cbSize;
	public uint dwMask;
	public short wNumbering;
	public short wReserved;
	public int dxStartIndent;
	public int dxRightIndent;
	public int dxOffset;
	public short wAlignment;
	public short cTabCount;
	[MarshalAs(UnmanagedType.ByValArray, SizeConst = 32)]
	public int[] rgxTabs;
	public int dySpaceBefore;
	public int dySpaceAfter;
	public int dyLineSpacing;
	public short sStyle;
	public byte bLineSpacingRule;
	public byte bOutlineLevel;
	public short wShadingWeight;
	public short wShadingStyle;
	public short wNumberingStart;
	public short wNumberingStyle;
	public short wNumberingTab;
	public short wBorderSpace;
	public short wBorderWidth;
	public short wBorders;
}


private void SetParagraph()
{
	PARAFORMAT2 prg=new PARAFORMAT2(); 
	prg.cbSize     = Marshal.SizeOf(prg); 
	prg.dyLineSpacing  = 200;
	prg.dwMask     = PFM_LINESPACING; 
	SendMessage(this.richTextBox1.Handle, EM_SETPARAFORMAT, SCF_SELECTION, ref prg);
}

private void SetParagraph()
		{
			
			PARAFORMAT2 fmt=new PARAFORMAT2(); 
			fmt.dwMask     = PFM_LINESPACING; 
			fmt.cbSize     = Marshal.SizeOf(fmt); 
			fmt.bLineSpacingRule  = 4;
			fmt.dyLineSpacing = 35;
			SendMessage(this.richTextBox1.Handle, EM_SETPARAFORMAT, 0, ref fmt);
		}

using(Graphics g=this.richTextBox1.CreateGraphics())
{ dpix=g.DpiX;
  dpiy=g.DpiY;
}

twipX=pixelX*1440/dpix; 
twipY=pixelY*1440/dpiy;

fmt.dyLineSpacing = pixelY*1440/dpiy;

fmt.dyLineSpacing = pointY*20; // вот он размер :)