If you’re hanging a chandelier over a table, make sure the bottom of the chandelier is around 30” above the table surface. Adjust accordingly for taller family members. For higher ceilings you may increase that height to about 36” - 42” above the table.
One rule of thumb is to measure the shortest width of your table and remove 12" for diameter of the fixture. For smaller rooms half of the shortest width of table would be your fixture diameter.
If you are considering a linear chandelier, the length should be between 1/3 and 1/2 the longest length of the table. Now that you know the size, it’s time to figure out what shape is right for your space.