First of all don't use "fill_parent" it a deprecated tearm, use "match_parent".
Secondly where do you want to place the half parent TextView in the parent?
As you are using RelativeLayout this action is a little harder, try to do it from code, some thing like this:
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int parentHeight = MeasureSpec.getSize(heightMeasureSpec);
int myWidth = (int) (parentHeight * 0.5);
super.onMeasure(MeasureSpec.makeMeasureSpec(myWidth, MeasureSpec.EXACTLY),
heightMeasureSpec);
}
reference: How to size an Android view based on its parent's dimensions