It depends on the material:
Wool styles (e.g. Wool Runner) feel snug at first but stretch slightly (about a quarter size) within a few days to adapt perfectly to your foot.
Tree styles (e.g. Tree Runner, Tree Breezer) are flexible from the start and keep their fit over time.
We recommend wearing them for 1–2 weeks to break them in. Wearing socks can speed this up.