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I have retina MacBook Pro with 256GB and a 1TB external HD.
That external 1TB hard drive I want to use for several things, one of them is to backup my system.
You can make the partitions any size you want. The larger you make it, the more backups you can save. TM deletes the oldest backups when it needs more space. You can also define what you backup in TM options. I don't like the methodology, but it takes the approach to back up everything.
You have to exclude things for which you don't need backups. For example, you may not want to back up the System itself. You may not want to backup some apps - where you can reload the installers.
Basically, you want to backup only stuff that you don't want to lose. So documents, images, music, work, etc.
If you have a 100GB of that stuff, you may want to make a 200GB partition. Then, other than a full backup of this data, TM has 100GB to make incremental backups of things you create or change.
I hope this helps. From your question, it appears that you know how to partition the disk so I did not go into that. Let us know if you need more on that.
You don't need to partition it. Time machine will manage itself within its own folder, leaving the rest of the drive up to you to use, without needing to segregate it into its own partition. Seeing as your MacBook drive is only 250 GB, and probably even less of it is full, time machine won't take up much space on the back up drive.
