Q: Custom horizontal picker on the page I'm relatively new to android development and even struggling a bit to pick the right terminology to use in the question. What I am trying to do it to create a custom control element that would look like on the picture attached. 

The idea is that a user can select the size by swiping either left or right, the line with the sizes would shift accordingly (40/M would shift to the position of 38/S if you swipe left). All the movements are happening within the element (neither size nor position of the element on the page changes). And the central (red) element is the selected choice.
Which direction should I look at to implement such control element? 

A: i found this library you can  use as per your requirement(see customView picker)
actually this is for date time but may be useful to you :) 

https://github.com/ai212983/android-spinnerwheel


A: Here is another one that is also for date, but can be modified to be used for your purposes. It looks very polished. 
Edit: Adding link to the library
https://code.google.com/p/android-dateslider/
