That'd really depends on how the party intends to make use of these PCs, their role-playing or tactical cleverness and the proclivities of the GM. Passing themselves off as new giant recruits hoping to join the growing army or possibly assassinating/impersonating/replacing a higher-up could wield amazing results with some successful die rolls as long as the GM likes the notion. The giant PCs could possibly end up with free-reign of most areas, invaluable tactical insight and possibly even giant allies working for them if really lucky.
After recoinoitering an area, making contact, winning over giant NPCs, the PC giants could possibly lead NPCs into carefully orchestrated ambushes or assassinations with minimal risks to the party; leading to huge tactical benefits as they gradually work their way through the giants' ranks. Silence spells, illusions and other means could be employed to keep the army's forces in the dark for quite awhile as the party slowly whittles away at it. The giant PCs could further mislead and confuse efforts to stop what's happening by spreading disinformation; throwing blame on one faction or another, weaving conspiracy theories and prodding the giants to either mutiny or disband. If truly daring, they could go so far as to try and seize control of the army for themselves; leading their giant forces against other giant enclaves in the following modules. The sky is the limit.
It really all depends on the play style of the group and especially the GM. Some GMs would encourage the guile of the players and reward their cleverness and good role-playing and see the PCs efforts as fun. Others would see it as a silly and unwelcome distraction from the tradional and straightforward kick-in-the-door style of gameplay; perhaps playing along for an encounter or two before bringing it to an unglorious end as the PCs' ruse is revealed and thwarted. It also depends on how much the other PCs are into it. If the non-giant PCs are unwisely sidelined while the half-giant and disguised wizard are out and about chatting up every giant in the valley, they'll quickly grow frustrated and less cooperative. To be successful and fun, everyone has to be on board and involved somehow.