I've just completed prince2 and I'm wondering what is considered a project in day to day work. I work as a software developer, so everything i do created business change. At what point do I assume that "this is a lot of work, I need to set up a plan, tolerances, stages with my manger". Is it more then one weeks worth of work? More then a month? Only when involving external parties??
I think this question is more about degree of formality and rigor. No matter the size or complexity of a project, we plan, we buy, we schedule activities, we think about and monitor our risks and issues, we may seek help, we use tools, we track and control and we finish up. What changes with size and complexity is the degree formality and rigor used. And formality and rigor come down documentation and reporting and the various PM tools one might deploy.
The question really is, when do you start the formality? As Richard suggests you could define the size of project via length of duration or money spent or some other scale. The benefit with this is you could expect a good degree of standardization across the organization. Other times, it might come down to "you'll know it when you see it." As complexity rises, the number of stakeholders increase, conflict is present, the environment is of high risk, you are little worried for your job, you can almost sense the gravitational pull to document, track, report, and use more and more expensive but high quality tools to help you.
I personally steer clear of heuristics that might define the "minimum requirements" to do something. Instead, I prefer the deploy what you need when you need it mentality.
Your company will probably have set tolerances for BAU / Small Change / Small Projects / Large Projects / OMG we're doomed projects
If not then defiantly it's worth setting this guidance up. Discuss with your management and any other PMs and if you have it a PMO in the company, what governance the senior mgmt team need for projects depending on their estimates. In my current organisation it's
<5 Days - BAU
<30 Days - Small Change
<100 Days - Small Project
<300 Days - Large Project
Now it's not just based on this.
You will have different tolerances dependent on (as you mentioned) working with external suppliers, or is the change dictated by an external body (key client or Government), does the change impact more than one of your systems or is the system a critical system which if it falls over your Sales team start throwing themselves out of the 5th floor windows
Hope that helps
Check the chapter in your PRINCE2 text regarding tailoring of the method. The key considerations are the complexity of your project and it's criticality to your organization.
The other thing to keep in mind is your own role on the project. As a developer, you should focus on appropriate tailoring, in conjunction with the Project Manager, of components that directly touch you. The most important piece here is the Work Package, but also includes things like the Product Description, Product Status Account, configuration item records, any kind of team plan that exists, expectations on delivery of checkpoint reports, etc etc