what would you do

If you were going to build a room based off a 400watt HPS with the possibility of taking it up to 600 or 1000 watt HPS later on what size room would you build?

Well to be clear, this will be a "closet" inside a room... both being built from scratch.
And I won't have to stuff any extra crap in there. I'll have space for any carbon filters, etc elsewhere. Basically, the light and the plants in a closet... what's the ideal size?

Honestly as big as you can make it in the closet my man. Space is always an issue indoors and light coverage you might have to bust out your old calculator to do some math. But the taller the ceiling the better obviously and I would recommend Indica dominant plants they grow squatter and thicker. Why do you feel you need all that filter shit my man. Basically if I know a little more about the situation I might be able to help more. I am a outdoor grower converting to hydro check out my first grow https://www.rollitup.org/dwc-bubbleponics/279092-dwc-bubbleponic-grow-cheese-white.html

ceiling will be normal height 8 feet

I don't necessarily HAVE to filter... I'm just saying I can put that elsewhere... or a CO2 tank.. or my reservior... anything like that I will have space for outside the closet.

it's mostly the light coverage...

No point to say what strain I'm growing because I don't plan on sticking to just one...

Inside this space I only want light and plants... and maybe a little extra free space to expand to a bigger light later on