Q: I only have 23 GB on my hard drive now. I realize that my hard drive is big enough, but I want to use Time machine to back up my pictures, etc. in case my hard drive crashes...
A: 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.