How to resize select layers to specific inches Photoshop CC?

carrene  wrote If I remember correctly you use to could resize an object by a specific %....  how do you do that now in Photoshop 2018?

Object is vague.   There are 3D objects,  There are Smart Object layers,   There are object in images. The open Photoshop document is a document object.  Photoshop can resize many object. All Photoshop are not resized the same way most or all can be resized by some percentage.   In your tile you specifically stated Layers plural,  All layer are not the same a background layer can not be resized and Smart object layer have an associated transform.    You can Target more than one layer and resize all the targeted layer with a single transform. They will be resized by the percentages you set in transform from their current size.   Transform will edit Smart Object layers targeted associated transform  and calculates  what its actual  percentage need to be from its current transformed size.

Here I transform three layer by 50%.  The top layer is a large smart Object layer that was already transform down in size. After the 50% transform you can see now its associated transform in near 25% not 50% it was at 50% before the 50% transform,

Make a Selection. If resizing entire layer contents and not the Background layer, it is not necessary to make a Selection. If Background layer, trash the Background Layer Lock.

Edit > Transform > Scale

In the Options bar: W  and link to H to maintain proportion). Enter % value

2nd method is Press the Alt+Crtl+i to show currently image size settings.

or go on Image then select image size.

Hi, Click the move tool and select the layer then make sure you enable the show transform controls option. Then click on the transform lines showing around the object and then upper side you will see the settings to change it.