We have measured total vocal-tract (VT) length, lip-tube length and glottal height during vowels on X-ray film data of short French utterances [1]. VT midpoints are determined by progressively fitting circles along the VT-length from the glottis to lip opening. The VT-length is obtained by summing up the distance between adjacent midpoints. Lip-tube length is defined as the distance between the incisors and the lip opening along the midline. Results show that the range of VT-length variation is 3.2cm with the average VT-length of 16.4cm. The cause of this large range appears to be the combination of the vowel dependent VT-length and prosodic position that influences on the glottal height. For example, during a vowel at sentence final position, glottis goes down with falling intonation or up with rising intonation corresponding to, respectively, VT lengthening or shortening. The lip-tube lengths are little affected by prosodic position and exhibit a clear vowel dependency. The prosodic influences manifested on the glottal height are not compensated but rather expanded in the VT-length, yet maintaining the characteristic vowel-dependency. This suggests an underlying mechanism to maintain a uniform stretching/compression of VT-length, which tends to hold the phonetic value of a vowel under large VT-length changes.