My MBP itself has 2TB storage and part of the reason for getting it was because the 2TB version is meant to be slightly more performant. Plus although it was fine to begin with I ran out of disk space on my 256GB MBA pretty quickly (and had to rely on external drives for large source files) so I wanted to ensure I avoided that with this Mac.
With regards to my external disks, they are used for Time Machine backups, which gives you multiple snapshots of your computer at different times (so bigger is definitely better for these).
I actually have two standard 1TB external HDs to, which I used periodically for Time Machine (but they are not always connected - and kept away from the computer).
500GB is probably enough unless you do video or graphical work and don’t want to use an external HD 
I have one of those too! And a scanner, tho it is rarely used now.
I have a couple of those too 