The critical exponent ν of the localization length in the integral quantum Hall regime can be measured directly using small Hall-bar geometries with different sizes. We obtain a universal behaviour for the three lowest Landau levels. This is in agreement with the universality prediction of the field-theoretic approach to the metal-insulator-transition in the quantum Hall effect. The value of ν=2.3±0.1 agrees with recent numerical studies for the lowest Landau level. We review recent experimental findings on the basis of these results and discuss the situation in Landau levels where spin-splitting is not resolved.