Use the measuring tool on the map edge tool to measure a feature instantly. You can either measure the length or the area size of a feature

You can use the draw tool and plot feature layers using the tool on the top right. In this way, you are not only getting the value of an individual feature, you can also get a sum of the features belonging to the same layer. Moreover you can even save these features into a project so that you can access it anytime anywhere online.

It's similar to the methods above. But in this way, you can add the dataset into the project first, then you can use the same tools on the top right to measure. This allows you to do the measuring later on.