UniQlo is a good store since it's a chain and there's usually one in every major metropolitian area.
At Dongdaemun, the Migiore (sp?) mall has a large size (women's large in US sizes and above) store somewhere on the 3rd floor, I think. It's right next to Doota. Not actually that close to the subway exit. It's been a while since I've been, but I think you have to take exit 9, Dongdaemun Cultural and History Park (not the plain ole Dongdeamun on the blue line). Er, it's been a while since I've been, so I can't give you good directions since it's not a simple matter of walk straight and turn once or twice. Hope someone else can fill you in.
I usually have no problem finding shirts, since a popular style is overlarge and drapey and that means perfect fit on me. Jeans and pants, however, I would suggest looking at the clearance racks: usually, it's the largest or smallest sizes they're trying to get rid of. I've found a lot of 30~32 sizes that way, and at an absolute steal!