Here's an image of my current setup which might help you out.

How large of an external do you think I'd need?

I was thinking of using RSYNC or GSYNC with these exclusions: (see the 2:04 mark)

https://www.youtube.com/watch?v=oS5uH0mzMTg

Post away, if you have anymore questions.

What is your backup strategy. Do you back up only user files and system configuration or you want also back up the whole system? I personally use rsnapshot and I do not do full system backup. This strategy has been working best for me over decades. IMHO considering how low is the risk of whole system being lost due to some drive failure I can afford spending a little time rebuilding it using saved configuration. Yet, this means much smaller and easier to handle backups.

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I was thinking of daily backup.

I was thinking of doing a backup with these exclusions.

The exclusions not withstanding, I was looking at doing a whole system backup, every day.

I couldn't find any information on rsnapshop on youtube, so there was a delay in my reply...

rsnapshot
It makes use of hard links, and I love that.

If you are planning to use rsync or other method without compression it will use the same space as the size of data being backed up although if you are planning to use rsync or other file by file method you should also account for allocation unit size of the target filesystem which in case of large number of small files may waste significant amount of space. If on other other hand you are planing to use dar or other tool which uses compression, stores all data in a single file and generally manages space more efficiently the size is less predictable and it's better to run a mock backup through pv and into /dev/null to measure its actual size.

I did look at rsnapshot, but the problem I found with it was my level of experience was not commensurate with using a command line only backup system - at this point in my journey into Linux.

If I use compression, do you think a 1 TB drive would be large enough? (I presume you've seen the image of my partition table.)

Thanks for the help...

What is this?

Compression only shows gains when the data/files is/are able to be compressed. There are various types of compression for different data. The more you compress, the slower it will go.
Some backup plans have a sort of test to tell you what it might end up with.

Yes, for instance there are special compression methods for audio and video, it is unlikely generic compression can add anything to those.