any size chart to refer?

No, but it runs true to size. I am 6’1 and about 200lbs with an athletic build and I ordered a large. Fits me like a glove.

Can you wear these comfortably under work clothes?

Yes you can. This type is very soft lycra spandex with light fleece inside. It fits conform tight but no sense of constriction feel. It should last better than store bought lycra types under wear garment. It has no chamois but sill can be worn first under cycling shorts. Nominal rides hardly notice no chamois on the average and disclaimer of how accurate like riding with regular underwear.

Would these be a good alternative to ua cold gear? looking for the same outer texture.

It depends how cold you're talking about. I can only say that for someone who ordered a size that was perhaps slightly too big by one size, with things a little loose, it kept me decently warm in 43 F (6 Celsius) weather where I was just wearing a pear of shorts on the outside, and a t-shirt over the top. It probably was sufficient for roughly 30-45 minutes of being outside in that sort of weather before the cold started to pierce through a bit more but nothing unbearable. I would say maybe after an hour or two though I'd probably start to feel a shiver but can't say that from experience. I don't remember the humidity but if it helps those temperatures I mentioned were from when I wore these back in December while in Las Vegas. Hope that helps.