What is the maximum capacity for a Time Capsule's hard drive?
Nobody seems to know what the official drive size limits are, but I just connected an 18TB hardware RAID to a 1st generation Time Capsule, and it works. To duplicate my setup:

Configure a 4-drive hardware RAID using this enclosure:

https://www.newegg.com/sans-digital-tr4utbpn/p/N82E16816111149

(I used four 6TB drives and set it up as a RAID 5)

Connect the RAID enclosure to the SATA connector on the Time Capsule's logic board using this cable:

https://www.newegg.com/p/N82E16812119251?Item=N82E16812119251

This Time Capsule has pretty much been gutted over the years, so the only thing left is the logic board itself. It's powered by a 2A 5VDC external brick, WiFi and routing are disabled. It's just a hack NAS at this point...but it handles an 18TB drive.