I assume you are asking about fitting a watch movement available on the market like the ETA 6498 that you had asked about in your email into a case and not actually making the entire movement such as the gears and all other components yourself. If I’m wrong please correct me. If you already have a case and would like to find a movement that will fit it you will need to determine all pertinent case dimensions such as the inside diameter of the case, the depth to determine how thick a movement will fit, stem height and location, dimensions necessary for securing the movement to the case, movement ring etc. You can then either measure a movement to see if it will fit or lacking the movement you can look at the movements spec sheet to make sure there is sufficient clearance.
There are new cases available on the market that allow for different ligne sizes and some that are already designed to be fitted with specific movements (take a look here http://www.tztoolshop.com/page89.html ). You can use large spacers/movement rings to fit a smaller movement into a larger case but this is generally not what you find with mechanical watches. A larger case with a large dial may require longer heavier hands to sweep the entire dial than what the smaller movement may have been designed to handle.
Personally if I wanted to start a small line of mechanical watches or even a one off using the movements and cases (which I have never done so probably a better way to go) available on the market today I would determine what movement I wanted to use and then check many different case manufactures styles for that particular movement or give the case maker a custom design that I would like to have made and let them know what movement I would be using.
I hope I addressed your question Jay and didn’t take off in the wrong direction.
Please let me know if I missed the target!