Gmail was one of the first email service to give huge space (in those days) for free emails. Also the search of gmail is so powerful, it encourage us to just archive emails we dont need and due to this we also stop organizing our emails. But after years of usage and also now the same space is used for store our documents, photos etc. 15 GB of free space seams to be limited and chances are you will run out of free space soon. To find a email which is taking huge space due to an attachment is hard to search by default in gmail.
Findbigmail.com comes to our rescue. Its safe to use security point of you. Just give access to the service and in within minutes it will find all the big emails in your gmail account and label them appropriately, now you can check those labels (FindBigMail > 10mb etc) and delete those email which are not useful any more. I was able to free almost 50% of my space by deleting really old emails. Check out Findbigmail.com and do donate if you like the service too.
PS: Try to take a backup if you can by downloading all emails to your outlook or any other desktop email client if you can before deleting. Back is always a better option.