Does it stack with size?

[Omey]

If iam medium size so i am considered gargantuan or only huge? if i use Bracers of relentless might (epic item)
And bullybashers's gauntlets(legacy weapon)

Giant Bearing (Su): Beginning at 13th level, you can multiply your weight by 8, gaining a +2 size bonus to Strength, a –2 size penalty to Dexterity (to a minimum of 1), a –1 penalty on attack rolls, and a +4 bonus on grapple checks. You double your carrying capacity and can grapple Huge creatures. You do not physically become larger and do not gain a longer reach, nor do you take size penalties on attacks, AC, and Hide checks. This ability can be used at will and lasts 10 minutes each time it is activated. Caster level 10th.

Bracers of Relentless Might
These bracers grant a +12 enhancement bonus to the wearer’s Strength and Constitution. The wearer is treated as two size categories larger than normal (to a maximum of Colossal) for purposes of combat-related opposed checks that apply a modifier based on size, such as bull rush, grapple, and trip.
Caster Level: 20th; Prerequisites: Craft Wondrous Item, Craft Epic Wondrous Item, bull’s strength, bear’s endurance, enlarge; Market Price: 4,384,000 gp; Weight: 1 lb

[SCOTTEPIPPEN]

If iam medium size so i am considered gargantuan or only huge? if i use Bracers of relentless might (epic item)
And bullybashers's gauntlets(legacy weapon)

Giant Bearing (Su): Beginning at 13th level, you can multiply your weight by 8, gaining a +2 size bonus to Strength, a –2 size penalty to Dexterity (to a minimum of 1), a –1 penalty on attack rolls, and a +4 bonus on grapple checks. You double your carrying capacity and can grapple Huge creatures. You do not physically become larger and do not gain a longer reach, nor do you take size penalties on attacks, AC, and Hide checks. This ability can be used at will and lasts 10 minutes each time it is activated. Caster level 10th.

Bracers of Relentless Might
These bracers grant a +12 enhancement bonus to the wearer’s Strength and Constitution. The wearer is treated as two size categories larger than normal (to a maximum of Colossal) for purposes of combat-related opposed checks that apply a modifier based on size, such as bull rush, grapple, and trip.
Caster Level: 20th; Prerequisites: Craft Wondrous Item, Craft Epic Wondrous Item, bull’s strength, bear’s endurance, enlarge; Market Price: 4,384,000 gp; Weight: 1 lb
I think that you are considered huge (for bonuses, not physically). Bracers of relentless might doesn't increase your actual size, so for Giant bearing's bonuses you are considered as you were medium.

[Zancloufer]

You would be considered Gargantuan for any grappling or related combat checks. You would not gain the extra reach or the penalties to hit/hide/etc however.

So pretty much for grappling and similar opposed checks you would be gargantuan on who you could grapple and size based bonus, huge for who could grapple you.