So to change the radius of your area effect spell you reduce your dice pool by 1 die per meter of expansion/reduction. Simple enough.

Then the text says "Dice expended to change the radius of effect cannot be used in any related test, such as resisting drain for that spell". Is this meant to make sure the reader knows the dice are used up, or does it mean the same sacrifice is made from the drain test?

E.g. I cast manaball and hold back 2 dice to increase the radius by two metres. Do I have to subtract 2 dice from by drain resistance pool too?

No, you don't have to subtract dice from your drain pool.

IMHO that sentence is a copy/paste leftover from SR3. Remember, back than you had a Magic Pool from which you could spend dice either to boost the spellcasting test or the drain test. If you withheld Magic Pool dice from the spellcasting test to increase/decrease radius, those dice could not be used on the drain test.
But since the mechanic changed in SR4 this sentence is mechanically irrelevant.

-CJ