Using the Advanced properties, you can change the page dimensions, check the below code:

Note: a premium license is needed.

Page page = m_doc.GetPage(0);
Ref ref = page.Advance_GetRef();
Obj page_obj = m_doc.Advance_GetObj(ref);
int count = page_obj.DictGetItemCount();
for (int cur = 0; cur < count; cur++) {
    String tag = page_obj.DictGetItemTag(cur);
    Obj item = page_obj.DictGetItem(cur);
    if ((tag.equals("MediaBox") || tag.equals("CropBox")) && item.GetType() == 6) {
        int arraycount = item.ArrayGetItemCount();
        for (int k = 0; k < arraycount; k++) {
            Obj array_obj = item.ArrayGetItem(k);
            //if(k == 0) array_obj.SetReal(0); //set left
            //if(k == 1) array_obj.SetReal(0); //set top
            //if(k == 2) array_obj.SetReal(500); //set width
            if (k == 3) array_obj.SetReal(900); //set Height
        }
        break;
    }
}