How big of an external hard drive do I need for Time Machine?

Question

I only have 23 GB on my hard drive now.

Answer 1

All macs have already  a hard drive that can suport time machine

Answer 2

i would just get the biggest one

Answer 3

You will want about double of what you are gonna use. You may have 23Gb now but how much do you think you will have in a year. Now double it. You want the extra external hard drive space because Time Machine backs up many many copies, so it needs much more space.