Microsoft has launched Windows 8 not only for desktop and laptops but it is tablet ready OS too and in tablets you get limited space which cant be extended in most cases and if you keep installing apps on your tablet then your limited space will decrease and your tablet might become slow.
So if you wanna know how much space is used by your Apps in Windows 8 then you can do it, for this press Windows Key + C to open your Charm Bar, now click on Settings and then it will open all Settings for you.
Now click on Change PC Settings
And then you will get a new window on which you click on General options and then on right side scroll down and click on View App Size.
It will take some time to calculate all installed app size and then once it determines then show you details of each and every installed app.
And then you can determine which app you want to remove or uninstall if you are running out of space on your Windows 8 machine.