To get the size of small work pieces by noncontact means, a new method is proposed in this paper which is based on the invariance of cross ratio with noncalibrated CCD cameras. After the images of work pieces being remotely acquired by camera and preprocessed, edges as well as some other features of the work piece will be detected and extracted. According to two orthogonal sets of parallel line markers with known spacing on the measuring table, the size of the work piece is obtained. Experimental results indicate that this kind of method used in length measurement system has similar precision to the traditional techniques which first of all must have their cameras calibrated. At the same time it can be simply operated and adapted to various situations. The measured length of work pieces ranges from 0mm to 500 mm and the measuring error is less than 0.4%.