Our 9v9 pitch changes size almost every week

I would keep the team's principles and adjust the distances. On a narrow pitch, the wide players may need to be more patient because the touchline arrives quickly. Support underneath the ball becomes important and switches may need to happen through two or three short passes. On a wider pitch, the team can stretch the opposition more and the far-side player has a bigger responsibility to stay connected rather than disappear. Before the game, I would give the players two observations: how much space is available outside, and how far apart the units need to be to support each other. I would not introduce a different formation every week. The pitch creates a different problem, which is useful learning. Let the players experience it, then help them recognise which distances and movements need to change while the main way you want to play stays familiar. I would adjust the support underneath first. The wide player can still stretch what space exists, but the nearest midfielder or outside defender needs to give a quicker return pass. If you bring everybody inside immediately, the pitch becomes even more crowded. Let the players discover how much width is available before changing the starting positions.