extent size MUST always be the same, in other words no matter how big, how fast is your table growing they should share same size extents therefore if you are using 8i it's suggested you use Locally Managed Tablespace because they ensure uniform extent sizes

You create several tablespaces

Code:
micro sized      64K
small sized       128k
medium sized   4M
big sized          128M
huge sized       512M
then just create your objects according your criteria, if you think objA is huge and grows fast you put it in 512M tablespace and so on

if you specify storage when you create a table then that storage would be used (tablespace storage would be ignored) but by doing so you are altering the uniform extent strategy