In photoshop trying to find width or height of something in pixels 

This is what u do 

Open the psd file 

select window-> info 
or
press f8

Select the info tab u can see with RGB & other info like XY  the location of pixel and XY the hight and width of the selection 
generally the unit will not be in pixel it would be in cm or some other unit

To change to pixels 

select the menu on the top right on the info window 

select panel options 

Change Moue Coordinates : Rule Unit to pixels 


Now u can measure the width of some thing or location of something using photosop