Or do runstats and check table size from the Control Center
Concerning growth:
Estimate how many new rows you get
pr. day and multiply it
by row storage requirement, i.e. the sum of space requirements
for all columns in your table.
If you have varchar columns the porcess migth be a little harder
because ypu have to assigne probalilties to the length of the
string inserted in each variable column
Good Luck