How much disk space do you actually need?
Disk space is the figure most prominent in hosting offers, and rarely the one that causes problems.
What a website uses
A brochure site with a few dozen pages fits comfortably under a gigabyte. Images are the weight, not text or code.
What grows unnoticed
Mailboxes on the same hosting, backups stored locally, and logs. A ten-year-old mail account often takes more than the entire website.
What matters more
Available memory and the PHP version determine your site's speed far more than space does. A slow site on a half-empty disk stays slow.
How to estimate
Look at the size of your current media folder and double it for growth. If you are starting from scratch, budget generously for images and modestly for everything else.
And if it is full
Clean up first, upgrade second. In most cases half of it is backups and unused media.