There's no way to trigger the dimension calculation with python, seq.update(data=True) doesn't do anything. Same applies to scene updates, frame_set, tag_redraw etc.
import bpy
import os
fp = os.path.expanduser("~") + os.sep + "testimage.png"
img = bpy.data.images.new("Image", 200, 200)
img.save_render(fp)
scene = bpy.context.scene
editor = scene.sequence_editor_create()
seq = editor.sequences.new_image("SeqImage", fp, 0, 1)
seq.update(True)
w = seq.elements[0].orig_width
h = seq.elements[0].orig_height
print(seq.name, w, h)
# SeqImage 0 0