When is it too big?

[BackPackHunter]

A few you you might have read a couple threads i wrote about a up coming build im planing. And the work needed in the room to make it fit...
The amount of work and money moving furnace, electrical panel, hot water tank and finishing the room, started to weigh on me, so I started thinking about ways to fix this...
what I came up with was build a new wall, and lose 6-8 feet in the living room..
I asked the wife her thoughts and to my surprise she likes that idea a lot more then the reno i had first planed ....
there is a window on one wall , so that could be a problem, I think its in a ok spot that wont affect the build, worst case is i build the new wall on a bit of a angle to avoid the window...

this new plan gives me a lot more room to play with ideas
i was going to build a 72x36x36 (and this will be the min size of it)
i still want a 36" deep tank, but might be able to go 42 or 48" wide and up to 10 feet long.... max would be 1200x36x48 (48" wide will depend on the window)

where do you draw the line ? how big is to big?

I dont see much cost difference in running a 72x36x36 - 1200x36x36
what are your thoughts on this?
at what size of tank do you got to up the glass thickness ?
was planing of going with 19mm

do you or have you have had a huge tank and wish you never went as big
whats the biggest tank you have had?

thanks for any input