Added a quick spec for the sizing on this. LMK if you need anything more. 

Deleted: Screen Shot 2018-11-27 at 9.01.18 AM.png 69.2 KB

	Screen Shot 2018-11-27 at 9.01.18 AM.png 69.2 KB View Download

Here's how it looks without and with a background/theme.

https://screenshot.googleplex.com/SjGT71VeutN.png
https://screenshot.googleplex.com/6eXirCU0knX.png
https://screenshot.googleplex.com/8xcd9KZvvgp.png

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a8dc15beb4c8c636a3721b521819b13b9823b162

commit a8dc15beb4c8c636a3721b521819b13b9823b162
Author: Kristi Park <kristipark@chromium.org>
Date: Mon Dec 03 19:38:18 2018

[NTP] Change fallback icon to a colored monogram inside a gray circle

Place the colored monogram inside a gray circle. This matches the
fallback style on mobile.

Screenshots:
https://screenshot.googleplex.com/SjGT71VeutN.png
https://screenshot.googleplex.com/6eXirCU0knX.png
https://screenshot.googleplex.com/8xcd9KZvvgp.png

Bug:  905437 
Change-Id: Ib26c04fc22e4eb50685c5de04300dd543a625cee
Reviewed-on: https://chromium-review.googlesource.com/c/1354318
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613194}
[modify] https://crrev.com/a8dc15beb4c8c636a3721b521819b13b9823b162/chrome/browser/search/ntp_icon_source.cc