How much code to include in a Physics paper? This question is related to Physics research papers, particularly one that will be coming up for my ongoing PhD research (4th paper in all).

A core part of the research is developing, testing and validating relevant self-written code.  So, my question is how much of the code should be included in the upcoming paper?  Should it be the whole thing, or a written summary of the main components/subroutines used?
If the goal of your paper is to show that your code solves a particular problem or performs a specific task, then it is your responsibility to prove to the reviewers that your code does what it claims to do. The easiest way to do this is to include the code as part of the supporting information for your paper. 

However, reproducibility—which is what you're asking about, to a certain extent—is a not-so-simple question when it comes to showing codes do what they're supposed to. For instance, a code that runs one way under one configuration may return a slightly (or perhaps _very) different result when run under another configuration. This doesn't mean that one result or the other is wrong—it just means that this behavior needs to be taken into account when evaluating the correctness of software.

One way to help this is to provide as much information as possible on your testing environment, so that any differences between the system on wich you were working and the conditions the reviewers and future potential users of your code will have can quickly be identified. This would be included, perhaps, as a text file that accompanies the code in the supporting information.