A hard disk is made of sectors numbered from 0 to the max.
dmesg gives, for example :
hdb: ST34321A, 4103MB w/128kB Cache, CHS=523/255/63
CHS means Cylinders, Heads, Sectors.
523*255*63=8401995 sectors of 512 bytes, thus the 4103Mbytes. This is only a logical map, it's not necessary what is written on the disk cover (except for the total size).
The true size of the sectors is of no interest for us given we don't want to modify anything but restore a previous state. For us, the default size given by fdisk is alright.
The size seen by the system is directly dependent of the work of the BIOS (Basic Input/Output System - the PC's ROM). The mode of the hard disk indicated in the BIOS is essential. On a new disk, it's better to use BIOS automatic hard disk recognition and say "yes". Anyway, any modification at this level may destroy all the data of the disk, so don't play with this without essential reason.
This is probably what your disk use, so don't be afraid.