When is it too big?

[MitchM]

I find the major issues to deal with on a large tank are lighting, heating, evaporation and cleaning.
The lighting is the most expensive part. No way around that. I changed over heating to a hot water PEX loop, and cleaning takes a lot longer, especially if it's a deep tank like you're thinking.
I keep the main tank covered but the sump is a major source of evaporation. Make sure your house mechanical systems can handle it. I have a HRV that is running most of the time.
I do 5% water changes weekly but I also keep a very low fish load, so I'm not finding any problems with smaller water changes.
My tank is 96 x 48 x 30. The sump is a 180g.