With the size of images being what they are in today's cameras, it is possible to print images of many different sizes. As far as editing is concerned, print size is irrelevant. So you specify print size in the print module. If you are using an outside lab to do your printing you can specify the size you want as you export the file that you will send to the lab.
An image does NOT have a size in inches (or centimeters) - unless it is printed. Size in inches is variable and dependant on the resolution in dpi/ppi.
But an image has a pixel dimension or pixel count, i.e. how many pixels are on the long/short side of the image.
For instance: If the image comes out of the camera with a pixel dimension of 2000 x 3000 pixels.
If you select a resolution of 300 ppi you arrive at a size of 10" x 6.66".
If you select a resolution of 200 ppi you arrive at a size of 15" x 10".
And if you select a resolution of 72 ppi you arrive at a size of 41.67" x 27.78".
Size in inches is the result of dividing pixel dimension by resolution in ppi. The resolution is not inherent in the image but is chosen by YOU. That is true too in Photoshop; In the <Image Size> dialog there is a box for the resolution and that determines the image size. Do a short test: In the <Image Size> dialog in Photoshop uncheck the box <Resample image> and then change the resolution. You will see the image size changing depending on the resolution that you type in.
So, the only thing that counts is the pixel dimension which will be displayed in Lr when you select <Cropped Dimensions> in >View options. See screen shot.