Google provides all design specifications in their design documentation, which is very rich and outlines every single element - its size, padding etc. Everything, including material icons and text fields, is based on an 8px grid so you're pretty much never left in the dark.
What about iPhone apps? Apple human interface guidelines are very vague and, in some ways, completely useless. That's why I'm reaching out to you guys. how do you position and size your design assets such as icons, bars, tabs etc.? I would like to be as precise as possible since that's just the way I design stuff. How are designs being put together in xcode? Is there anything I need to consider?