Where does GarageBand put its downloaded instruments and loop libraries?
As for total size, that is difficult to say. I got my GarageBand instrument, sample library and loop files from an iLife DVD installation a few years ago, rather than downloading them from the Mac App Store, which is the only way to get them today.

It's my understanding that when you first purchase and install GarageBand from the Mac App Store, you do not get all the musical instrument sample libraries and loop libraries that are listed in GarageBand's indexes. If you try to select an instrument or loop from an index and it is not found on your Mac, then you are prompted to download another large section of the libraries. Then there are the music lessons, which you must purchase one at a time, download and install.

My basic installation for GarageBand instruments and music lessons in ~/Library/Application Support/GarageBand is 2.74GB.

The loops for GarageBand are stored in ~/Library/Audio/Apple Loops/, and my installation is 629.9MB. However I do not believe that I have downloaded the whole set.

At an Apple retailer where I have worked, their installation in 


~/Library/Application Support/GarageBand is 14.57GB, 


and their installation in


~/Library/Audio/Apple Loops is 32GB.


This includes the GarageBand-compatible instruments and loops that you get when you purchase Apple MainStage, which you can get from the Mac App Store for $30. However, this does not include all the music lessons you can purchase within GarageBand.

Furthermore, you can purchase all sorts of Audio Unit-format effects, musical instruments, sample libraries and loop libraries from third parties that you can use within Garage Band or Logic, so you could add many hundreds of gigabytes of additional material if you want to.

It is worth mentioning that electronic musicians who work with large libraries of virtual instruments, samples and loops in GarageBand or other similar programs invariably move all such libraries to external high-speed disk drives so as not to slow down their systems with too much read-write activity on the system hard drive. 

If your work with GarageBand is simple enough--not too many tracks in a mix--you can work with just your Mac and system hard drive. But if you discover that Garageband is glitching, hiccuping, or failing to record or edit correctly, or you get errors about input and output being too slow, you need to incorporate additional high-speed hard disks into your system.

It's a general principle that you need at least three hard drives: 


your system drive inside your Mac, which holds Mac OS X and the
GarageBand app 
an external high-speed hard drive for sample
libraries, virtual instruments and loop libraries 
another external high-speed hard drive for recording and mixing audio tracks.


All of these have to be interfaced with a high-speed interface such as FireWire 800, USB 3.0, or Thunderbolt. USB 2.0 will not work, because it is not fast enough for composing, recording and mixing music and audio.