The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/97e9895c976520ff7fc128d0a43d37c7f4e4a59c

commit 97e9895c976520ff7fc128d0a43d37c7f4e4a59c
Author: Amirhossein Simjour <asimjour@chromium.org>
Date: Fri Feb 02 21:19:57 2018

VR: Fix the size of the JS dialog

The size is fixed to follow the design

BUG= 808560 

Change-Id: If63ebbd676d62acd4cbc46081551d73835ced49c
Reviewed-on: https://chromium-review.googlesource.com/900055
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Amirhossein Simjour <asimjour@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534156}
[modify] https://crrev.com/97e9895c976520ff7fc128d0a43d37c7f4e4a59c/chrome/android/java/src/org/chromium/chrome/browser/vr_shell/VrDialog.java

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/aff665fc085f8acf570abdab73098d833fdb8b7c

commit aff665fc085f8acf570abdab73098d833fdb8b7c
Author: Amirhossein Simjour <asimjour@chromium.org>
Date: Wed Mar 07 22:30:12 2018

VR: fix the position of dialogs

Backplane of the hosted dialogs are moved to align
with main content. As a result the dialog itself
will be in the center of the web content.

BUG= 808560 

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_vr;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I6addedbfef905c51e11d5a6a5c95f889775a2078
Reviewed-on: https://chromium-review.googlesource.com/953247
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Amirhossein Simjour <asimjour@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541613}
[modify] https://crrev.com/aff665fc085f8acf570abdab73098d833fdb8b7c/chrome/browser/vr/ui_scene_creator.cc

Verified on 67.0.3368.0  it seems better positioned for the user.
It is not in M-66 yet.