Gmail is a most popular choice when it comes to free Email service,you can use all the features however you have a limited amount of storage space(Approx 7GB).You need to pay google if you need additional storage.
For newbie gmail users storage wont be a problem as it needs more years to get your in-boxes full.For those who have heavy in-boxes you need to clear up the emails occupying large space.Its very difficult for you to scan entire inbox one by one and delete unwanted emails.
www.findbigmail.com is a free application which you dont need to download and install, which scans your gmail inbox and applies the labels for emails which are greater than 2MB.So it will be much easier for you to get a list of larger emails and delete those unwanted ones.
additionally it applies labels to emails having sizes greater than 500kb,100kb.
After cleaning up the space you can use priority inbox feature to sort out the Gmail inbox and become a power user.