Electronic file size is the measure of space an upload, attachment, or document takes up on a computer (not necessarily the same thing as a word or page count). File sizes can be measured in small kilobytes (KB), large megabytes (MB), and beyond--just 1MB can hold 500 pages of text! It is increasingly important in most transactions today to be familiar with file sizes as a courtesy to clients, colleagues, and others when you send documents, pictures, etc. to their inboxes. Everyone has (and pays for) limited storage, and items you send to others take up some of their storage.
Some scanner machines are set by default to take very high resolution scans of your materials, producing very large file sizes no matter how many pages they contain. If you scanned your file and it is now several megabytes large, your scanner settings can be adjusted to be lower-resolution--talk to someone more familiar with IT to help show you the settings, and you will save yourself and others memory/storage in the future. For documents you already scanned, you can use an online compressor like https://www.adobe.com/acrobat/online/compress-pdf.html to make them smaller now. And, for other types of files, there are many learning resources like this online where you can read about best practices for creating files of manageable sizes.