Experimental evidence is presented for three probe-size effects which influence the interpretation of Langmuir probe measurements in tokamak scrape-off plasmas. Small probes can give erroneously high values of plasma density. Large probes can only measure reliably the average energy of the high energy tail of the electron distribution. Insertion of a large probe into the scrape-off plasma can strongly disturb the plasma necessitating special analysis procedures for recovery of the unperturbed plasma characteristics.