The Blob.size property returns the size in bytes of the Blob or a File.

API Files Property Reference Référence Web API

The Element interface represents an object of a Document. This interface describes methods and properties common to all kinds of elements. Specific behaviors are described in interfaces which inherit from Element but add additional functionality. For example, the HTMLElement interface is the base interface for HTML elements, while the SVGElement interface is the basis for all SVG elements.

API DOM DOM Reference Element Interface Reference Web API