Export an image with a fixed width (height variable, depending on dimensions)

I'm a long time user, but have never needed to do this, and I feel like a rookie. How do I export an image from LR Classic 2024, defining the width in cm/inches, setting resolution to say 150, and letting the height vary as per the dimensions of the image?

I've tried exporting, specifying my width, leaving height blank, setting resolution, but it seems to default to setting the height to what I've specified. I can't get it to take the width.

I'm know I'm doing something sillly, but can't for the lfie of me figure out what.