Use the Rectangular Marquee Tool. In the options bar at the top, change Style from Normal to Fixed Size, then enter 340 px for width and 210 px for height. After that, you don’t drag to size the box—just click where you want the selection’s top-left corner to start, and Photoshop creates a selection at exactly that size.

If your goal is to crop instead of only select, you can also use the Crop Tool and enter the target dimensions in its option fields. That keeps the crop constrained to the specified size while you position it. Be aware that cropping this way can resize the cropped result to those entered dimensions.

Another simple approach for a fixed final image size is to set the canvas size to 340 × 210 and then move the image layer until the desired area fits within the canvas.