<SPAN style="FONT-SIZE: 18px">android:textStyle="bold"</SPAN>

<string name="styled_text">Plain, <b>bold</b>, <i>italic</i>, <b><i>bold-italic</i></b></string>

TextView tv = (TextView)findViewById(R.id.tv);
TextPaint tp = tv.getPaint();
tp.setFakeBoldText(true);