Is this shirt true to size?

It is described as a relaxed fit, so it wears roomier through the body than a slim or regular cut. Trim builds often size down; those who want room to layer keep their usual size.

Is the cotton heavy like Carhartt work shirts?

No. It is described as soft, lightweight cotton for warm weather, closer to a summer button-down than Carhartt's canvas or flannel work shirts.

Can it be worn to an office?

In a casual workplace, yes. The button-down collar keeps it neat, but the short sleeves and relaxed cut make it informal rather than business dress.

Does it work layered over a T-shirt?

Yes. The relaxed fit leaves room for a tee underneath, and the short sleeves suit open-over-a-tee wear in warm weather.

How should I care for it?

Cotton like this is machine washable; wash cold and hang or tumble low to limit shrinkage. Expect some wrinkling, so iron or steam for a crisp finish.