If you have a Viewport with BorderLayout, a grid and a form all in one file it certainly is not the big application, right?. If you have tens of windows each with a grid, form or border layout in it in tens of files it certainly is the bit application, right?

(Germans have very nice word: Jein which is combination of Ja = Yes and Nein = No.)

The answer to both above statement is Jein. When the application becomes big, then? The answer is simple: It becomes big when you feel it is big. It is the point when you start to have troubles to orient yourself in number of files or in you have troubles to find a specific place in one file, when you cease to understand relations of components, etc. I am writing you here but imagine when a 2-3 grades less experienced programmer starts to have this feelings.

We can safely state that each application is big as also a small application deserves to be well written and it may likely become really big as we start to add new features, write new lines of code, new CSS rules, etc.

The best and the safest state of the mind at the start of a new application is: I’m starting the big application!