On the Chaos Warlord's card, there appear to be tiny army units at his feet. Are these really small minions of Chaos or is the Warlord a giant?

Nope. He's normal space marine size. So probably close to 8 feet.

Just checked the card and it really has small CSMs at the warlord's feet. Although they are in a haze, so the only explanation I have is that it's a sorcery of some kind, maybe just a vision of a battlefield like on a hologram.

I think it's just the artwork being abstract in presenting the idea that he is controlling the fates of others. A sorcerer of Tzeentch being an overreaching influence on a battlefield. At least, that's the impression I get. Interesting that you see smaller Tau in Shadowsun's artwork as well, but that one manages to convey the impression that she is standing on higher ground.

Yep its just the art, as mentioned he is probably 8 feet without the helmet but nontheless not a Giant being.

Do note that all cards have a vague piece of art behind them altough I am a 100% sure this is just there to create the idea of a commander/Warlord leading it's troops, not a scale reference.

Then again, to the average human in the 40k universe, a space marine IS a giant.

Then again, to the average human in the 40k universe, a space marine IS a giant.

True, altough unlike the other giant beings in this universe the average human sees a space marine as a heroic knight, unless he/she is a cultist but then sees the chaos space marine as a hero.