The measure tool in AGOL is very good to measure length / areas of your wish.
This idea has been merged with Calculate geometry in AGOL to find the area or length of a polygon or line.
But there is really no way to create a polygon / line with the length/area statistics.
Field calc is available, but no way to calculate the geometry.
Either if you can create a feature of from the Measure tool, or be able to calculate the Area / Length from a geometry service.