Do you need to choose your effect at the beginning
the game, or can it be chosen mid game?

When in doubt, consult the source

Prerequisites: {starburst}, Avengers TA

Choose a character.

Once per game, choose one - the character has the {giant} damage symbol; or the character modifies its defense value by +1 against ranged combat attacks. The chosen effects last until the end of the game.

Doesn't say anything about the beginning of the game, just once per game. So, if you want to wait and see what will be more beneficial, go right ahead.

And also...can you turn off the feat if you
choose to be a giant, and want to go
back to normal size?

All feats can be turned off unless they specifically say otherwise (like the clarification on Coordination).

Since Pym Particles does not say it is non-optional, it can be turned off. Whether or not this is the way GD meant for this to work remains to be seen, but this is what we've got for now.

Here's a question that hasn't really been answered, but how does the timing work for the feat for when you can choose? Let's say the opponent makes a ranged combat attack and needs a 7 or better to hit, and they roll the dice and roll a 7, can you then declare you are choosing the +1 to defense against range option and thus avoid the attack? I know that seems kinda sleazy but is there any rule to prevent you from doing this after the dice have been rolled? There's no action that is required to choose the option, so there does not seem to be any restriction of making the choice in mid-action unlike some free action powers.

feats are optional on a per action basis.
I would say when they declare the attack the feat is in effect with no bonus, or it is not active since you have not chosen.
Since the action has all ready been declared and started you would not be bale to choose/activate the feat.

Ok, that makes sense. I figured no matter what it wouldn't be allowed, I just wanted a good explanation to say why it wouldn't be allowed.

I see no reason that you could not make your choice in response to a declared attack...like canceling a power or changing WC TAs.

If the attack has been declared and rolled, however, I would say that it is too late for making the choice in order to impact the attack.

Seconded.

There really isn't any specific rule to back this up (so take this for what you will), but since the few "interrupt" powers/feats/etc that do exist (Probability Control and Opportunist, for example) specifically say that they are applied to or otherwise affect the outcome of the die roll, and Pym Particles does not, I would also lean towards Pym Particles having a timing more akin to wildcards.

If all feats can be turned off unless they specifically say otherwise, how about: Fantastic Four ATA feat, Thunderbolts ATA feat, Squadron Supreme ATA feat, Life Model Decoy...

In these ones that I mentioned, I could see a situation where you would like to turn these feats off and either use the original Team ability, or in the case of life model decoy- be healed. If, as you say, All feats can be turned off unless they specifically say otherwise, why can't these be turned off as well, if they do not say otherwise?

Thanks for indulging me.

I just read under Section 7: Feats, General Clarifications, of the Players Guide that ATAs are not optional, so strike those from my original question. I Still don't see anything about Life model Decoy though.

Life Model Decoy is 'kinda optional'. You can choose to not use the damage reduction part, but you cannot choose not to use the 'cannot be healed' part.

That makes sense to me. Can you point out the wording in the Players Guide or rulebook though. Just wondering if that feat has been specifically addressed yet or not.

i agree but the OP was in regards to activating it after the dice were rolled.