Our analytic formula for ionization potential (IP) of a metal cluster of given radius, R, is adapted to small size clusters with number of atoms n < 40. The radius is defined as the distance between gravity center and the most remote atom surface. R was either calculated or determined experimentally with relative uncertainty of 3%. We have found that R(n) significantly fluctuates around 1.30 n(1/3) for n < 40. The calculated IP values for Y-n and Ce-n small clusters indicate the valence transition from Z = 3 to Z = 2 at n = 6 and 3, respectively, whereas Z = 3 may be considered for Pr-5 and perhaps for Pr-6 <(c)> 2001 Published by Elsevier Science B.V.