If you want to get the width and height before your activity has added it to its view hierarchy, measured and layed it out you will have to call measure() on the View yourself before calling getMeasuredWidth() or getMeasuredHeight().
As the measured width and height are set after measure has been called (which in turn calls onMeasure()) they will return 0 before this point. You will have to supply MeasureSpecs to measure(..) that will vary depending on your needs.
The MeasureSpecs that allow the children to impose any constraints themselves look like
int widthMeasureSpec = MeasureSpec.makeMeasureSpec(*some width*, MeasureSpec.EXACTLY);
int heightMeasureSpec = MeasureSpec.makeMeasureSpec(*some height*, MeasureSpec.EXACTLY);
Important point. (has been changed in new API) The width passed in above can either be explicit px or ViewGroup.LayoutParams.WRAP_CONTENT or ViewGroup.LayoutParams.FILL_PARENT.
Its also worth noting when your view is measured in the actual destination hierarchy the MeasureSpec that will be passed to measure() will be configured based upon the containing ViewGroups layout logic. It may be called more than once if the first measure vals are not valid when considered in the context of this parent viewGroup / its own layout constraits / the constraints of any sibling child-views. Without waiting for the viewGroup to call measure() before implementing any logic dependent on the measured size it would be hard (depending on the situation) to get the final measuredWidth & height from the above solution, but in all the instances i have used the above technique it has fit the purpose, but they have been relatively simple. If you really do need to measure in context you should probably just do it after onLayout() has returned and explicitly set any changes then requestLayout() again.