Usually 1.5x the window width is the minimum recommended. For a window that closes, I usually use 2.5x or 3x the width of the window. My window treatment book says that for sheer fabrics, you should use up to 5x the window width (or 360" of fabric for a 72" window).

Since you want a contemporary look, you might want to consider roll down shades instead of sheers.

I was thinking about rolldown shades, but do you think it is ok to have those shades down at most of the time? The window itself is not great. Should I also try semi sheers? Thanks

These are old black metal windows. They just don't look very presentable. I am considering painting window the window frame white or very light gray I have on my walls, but the window frame has black rubber inside that's a little bit visible, so this may or may not work. I have most of my furnishings figured out. There is a multicolored rug (pinks, orange, blue and some yellow) and I already have a gray couch in there and have a modern media stand on its way. I was suggested that I Pick up shades from the rug for my window treatments, but I am not sure I am that adventurous and even if I try that, I first need to figure out the type of coverings that would work in my space. Here is a photo of bare windows when we moved in. Most of the decor, except for the rug, now looks different.

Heat or light are not really a problem. That's why I am leaning towards curtains and sheers specifically. They can still look contemporary, right? Do you think I would have my issue solved if I increase the volume on a 70 inch window from 100inch to 200inch?

Sheers need 2.5 to 3 times the width of the window to look right. Otherwise, they look skimpy. Use the rings for a more contemporary look. You can shorten them temporarily if the label indicates they'll shrink when washed. If dry cleaned, shrinkage is usually minimal.

So is it fine to hang 3 times the volume in sheer and nothing else (if heat and sun are not a problem)?

Not volume, width.

Yes they can look modern and be nice if that extra width is there.

I just tried increasing the width (moved curtains from one window to the other) and it looks too much. Perhaps it is because my curtains are a little thicker than regular sheers and they have a pattern. They are this https://www.crateandbarrel.com/briza-50x96-ivory-sheer-linen-curtain-panel/s184033).

In fact, they look pretty nice during the day. Just a little weird at night.

Perhaps somebody can suggest different curtains. I am also considering roller shades, but I love the idea of just having nice sheers or semi-sheers.

I LOVE YOUR OLD BLACK METAL WINDOWS :)

When closed, your curtains should have at least this much fullness:

Otherwise it looks like you've hung sheets on the windows.

Rebecca made me go back and look at your windows. Those black windows are perfect for this contemporary house, and your view is to die for, so why are you covering them at all? There's a reason the previous owners left them bare. Unless you need sun protection, leave those windows nekkid.