If you are using a Windows laptop right click on the Windows icon at the far left of your task bar at the bottom of your screen. It looks like for boxes (or window panes!).
Click on 'Disk Management' and a panel will open giving you all the storage details of every hard drive you have connected to your pc, including your external hard drive.
The simplest way (for a Windows machine)is to:
1. Open 'File Explorer' (usually a Folder icon at the bottom of your screen)
2. Scroll down the left-hand pane until you see 'This PC'
3. Right-click the drive in which you're interested (e.g. C:, E:)
4. Left-click 'Properties' (the last in the box). This gives you the sizes you need.