plastron is their underside.. there are smaller but they're nigh on impossible to get hold of n most are wild caught off what i've heard. also if u can get them they tend to have higher care requirements
Plus horsfields have absolutely amazing personalities (i'm not biased honestly

)
[edit] i thought stars were roughly the same size. 28cm average size for adults i read, 20 cm average for hermanns n 25cm for horsfields, course this was about a year ago n everything else on the site turned out to be tripe..ooh forgot theres also the spur-thighed tortoise as well