Large (800 GB) Disk Image on Time Capsule as pseudo-NAS? I am considering getting a new Time Capsule (2TB) as a basic/pseudo NAS, in addition to a target for Time Machine for two macs. 

My macs each have a 500 GB ssd. Neither are currently even half full, but at some point down the road, I assume they'll each have 400 GB. I think I've read that Time Machine should have storage for at least 1.5x the amount of data being backed up, so 400 x 1.5 = 600, x2 macs = 1.2 TB needed. 

With the 2 TB Time Capsule, I'd have 800 GB available for NAS, but I need to LIMIT the space used by Time Machine, since it otherwise continues to grow, using all available space on the disk. But the TC disk can't be easily partitioned, so the solution I found is to put my NAS files in a blank Disk Image on the Time Capsule -- which prevents Time Machine from using that space. (See Can I set up quotas on my Time Capsule?) 

When creating a Disk Image in Disk Utility, the largest preset is 8.3 GB. But this would require an 800 GB Disk Image. So finally, my question...

QUESTION -- Is an 800 GB Disk Image (and this strategy in general) ok, or is it a very bad idea? Why?
This is a bad idea on a Time capsule. It is possible, but it will be extremely slow to set up and very slow in use.

See Pondini's Time Capsule Q3

You can use Disk Utility to create a custom-size disk image (normal, not sparse) and specify it to be 800GB size, but you have to create that empty 800GB file locally first, then upload it to your Time Capsule. The Time Capsule's disk and network connections are not built for speed.

Even if you reserve 800GB of space on your Time Capsule, there is no quota to shared the remaining 1200GB fairly between your users. 

For example - One user could repeatedly fill their SSD with junk, back it up to the Time Machine target on the Time Capsule, delete the local junk and download fresh junk to the SSD. They could repeat this until the 1200GB space is used up. Only at that point would their oldest junk be automatically deleted from their Time Machine history. The other user would not be able to prompt any clear-out of the junk, they would only be able to lose their own oldest material.

Shorter answer: Time Capsule is great, but it is a backup device not a NAS: it is too slow and has no NAS functions. Don't try to use it like a proper NAS. It is handy for storing a few shared files like a very very basic NAS can, but that is very limited.