Hi
If a project is headed for an enclosure, the first thing to do is to pick an enclosure. You then measure out all the mounting points and transfer that information to your PC layout software. Effectively, the board mounting process defines the size of the board. It may define edges that slide in slots. It may define holes that line up with mounting bosses in the enclosure.
If the project is something that will handle power, heat sinking is part of the process as well. You often will heatsink parts to a metal case rather than buying fans and heatsinks. The metal walls of the case will dictate the locations of your "hot" parts on the board. Again, the enclosure dictates the layout.
If the project has connections, they also will interact with both the enclosure and the layout. For most "stuff" bringing in bulk power at the back makes sense. Connecting instruments up on the front or back may make sense. Anything you need to adjust or any fault indicators need to go on the front. In most (but not all) enclosures, the connections and indicators are mounted at right angles on the ends of the pc board.
Some simple enclosure types:
1) A folded metal box: They have 6 sides, generally of aluminum and a flat bottom plate. The opening in the bottom is a bit smaller than the inside X,Y dimensions of the box. Connections all are out the "top" of the box if they are PC mounted or you hook them up with wires. They are pretty easy to heatsink things to.
http://www.budind.com/view/Aluminum+Enclosure/Aluminum+Chassis2) A die cast box: A cousin of the folded metal box. They are a bit better looking. They come in a few different configurations. The common ones have a full sized bottom opening. Otherwise they have the same issues as the folded metal box.
http://www.hammondmfg.com/dwg.htm3) An extrusion (tube) box: The body of the box is an aluminum extrusion. It makes up the top, bottom, and left and right sides of the box. You get plates to make up the front and back of the box. The PC board slides into slots in the extrusion. This makes it easy to have PC board mounted parts out the front and back. Heatsinking to the case is not so easy.
http://www.hammondmfg.com/sinkbox.htmFor most of what I do, I find the extruded boxes on eBay. I may spend a while shopping. I may get 10X more than I need. They rarely cost very much money at all.
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You might want to think a bit about what your "current limit" 317 does to the voltage regulation ...
Bob