Disk space, also known as storage or web space, is the amount of room on a server available to store the content (text, images, code, databases, etc.) of your website.
The web space can serve two basic purposes. In the first place, it allows you to upload user visible data (HTML files, image files, etc.) as well as user invisible data ( email messages, PHP files, database files, linked CSS files, JavaScript files, log files, etc.)
Most web pages are less than 100 KB in size, mp3 songs are typically about 4MB and most email messages are smaller than 5 KB, but this can vary as per the size of the attachements; if you are an architectural or legal firm, expect the attachments to be heavier in size than normal.
You should always start with the Hosting plan that meets your initial requirement and upgrade it as and when you need higher resources.