If you want to see the size of a specific file or folder, click it once & press Command-I. To see storage information about your Mac, click the Apple menu at the top-left of the screen. Choose About This Mac & click the Storage tab. For Time Machine backups, its always good to use a drive which has twice the storage capacity of your Mac.
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Make automatic backups

Mac comes with software which make backing up easy & simple. Turn on Time Machine & connect to an external drive. Time Machine will makes automatic backups of everything which you can later recover individual file or entire system from your backups.
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or you can choose other backup methods

Backup in iCloud

iCloud lets you store & access your photos, music & other files in the cloud. iCloud comes with 5GB of free storage.

Burn DVDs & CDs

If you have a Mac with built-in or external DVD drive, you can burn your files to CDs & DVDs to create backups.

Copy files to other storage device

You can also copy files & folders to an external storage device any time you want to backup.