They can occur on all scales and in all environments. A common misconception is that rocks can only fold when they are nearly molten. These outcrops tell a different story. Rocks like these limestones can fold even at the Earth's surface - provided they are given enough time. A fold like this could take a hundred thousand years to grow.
Folds can occur on all scales.
These folds from the island of Syros only have a wavelength of a few millimetres. To make folds the rocks must have a mechanical layering. If they are homogeneous they will just squash together without folding. You can try this yourself - using different types of plasticene. You can find out more about how rocks fold by clicking here .