I want to make a backup of my phone using itunes on my computer, how much space will I need?

Will I need the whole 20 gb my phone is using, or does it create an image that is a few gb?

You can choose to back it up to iCloud - unless that is full of pictures, etc - or you can see if there's enough room on your hard drive. Only you can find that info.

Itunes should do that automatically when you plug it in to your computer

Try it and see, you will get a message if there's insufficient space

Data is data. 20 gigs is 20 gigs. Unless you archive it.

Naw jus a few. It doesn't save your whole phone. Look at how much storage you used on your latest icloud backup. That should give you a pretty good estimate.

As big as you iPhone is, I'm using about 10Gb I have a 16Gb iPhone 6. Also my iPod Touch uses space as well.