If I have a composable that is [0, 0, X, Y] size big and I wanted to clip it to display (and also report its size as) only a small part of that, how would I achieve that ? Say my composable reports its size as 100 x 100 but its children/content have a 500 x 500 canvas to draw on and I can select what area within that 500 x 500 canvas is visible in that 100 x 100 window 🤔

Not sure I understand your question, but take a look on https://developer.android.com/reference/kotlin/androidx/compose/foundation/layout/package-summary#boxwithconstraints. It gives you information about current constraints through https://developer.android.com/reference/kotlin/androidx/compose/foundation/layout/BoxWithConstraintsScope