Oversized almost forces the race into melee. It's not required for a character with oversized to be melee focused, of course, but then a fairly large portion of your character sort of 'goes to waste'.
Your firbolg has this problem all around;
Terrain battle mastery should instead be a more generic bonus (such as "While in difficult terrain, you have a +2 bonus to damage rolls") - this gets rid of the reliance on thrown weapons
Oversized should be dropped in favor of something else. I don't know what, though.
I'd drop the damaging aspect of firbolg surge. Make it a psuedo-rage - spend a healing surge, maybe gain some temp hp, short-term bonuses to attack and damage, maybe a shift - that sort of thing, stuff that any class could utilize to the fullest.
Also, because of the Str and Con bonuses both being in the Fortitude defense, I'd add in a +1 racial bonus to a defense - I'd probably go for Will.
In the Elder, I'd get rid of the weapon bonus - It's REALLY bland. Removing it won't gimp the PP, either
Feeblemind is a little odd. I'd suggest something like "When you spend an action point to make an attack, if that attack hits, the target is weakened (save ends)".
You could easily make the 16th level ability Oversized. It delays it to mid paragon, and PPs don't suffer from being designed specifically for melee, the way races do.
Another thing to consider putting somewhere in there is threatening reach. You only get Opportunity Attacks against creatures adjacent to you, regardless of your reach. For that you need threatening reach.
Hurl rock seems kinda weak... I dunno. I'd have to do some comparison. Perhaps increasing its range or adding a push or prone effect would help? Also, since the character'll be in the thick of things, make it a close burst attack instead of a ranged - functionally the same thing, if you've one target, except the giant won't provoke opportunity attacks.
I DON'T like Elder's Advantage, especially as an encounter power. Perhaps something along these lines?
Daily • Stance
Minor Action • Personal
Effect: Each time an enemy makes an opportunity attack, you gain combat advantage against that creature until the end of your next turn.
Or something like that.
Make trampling charge a weapon attack. Also, I'd reword it to PREVENT OAs from whoever you trample. Something like;
Daily • Weapon
Standard Action • Melee weapon
Target: One creature
Special: Before the attack, you may shift up to your speed. You may move through the target's space without provoking opportunity attacks.
Attack: Strength vs. AC
Hit: 4[W] + Strength modifier damage, and the target is knocked prone.