The cooling temperatures of rectangular parallelepiped Bi and Bi0.88Sb0.12 polycrystals of various dimensions were measured at 298 K as a function of electric current in magnetic fields up to 2.17 T, where the magnetic field was aligned along the thickness (t) direction of the sample and copper current leads were soldered to the centers of two ridges along the thickness direction on two diagonally opposite corners of a rectangle with length L and width W. With this configuration, the temperature measurements were carried out at both end surfaces with the cross section (W×t). The thermoelement was not in contact with a heat sink. The maximum cooling temperature at the cooled surface increases with increasing magnetic field, but it depends strongly on not only the aspect ratio of the sample but also its thickness. It was found that geometrically optimum shaping of a thermoelement is very important for achieving high-cooling efficiency. A thermoelement made of Bi0.88Sb0.12 with optimum dimensions of L=15 mm, W=2 mm, and t=3 mm effects the high-cooling temperature of 4.2 K in a magnetic field of 2.17 T. As a result, the cooling temperature reached 74% of the cooling temperature of 5.7 K obtained with p-type Bi2Te3 of L=12 mm and W=t=4.8 mm which has a thermoelectric figure of merit of ZT=0.87 at 298 K.