Hi Peter,
The thumbnail size for the video is 320x240.
<?xml version="1.0" encoding="UTF-8"?>
<howcast version="0.1">
<video>
<id>180126</id>
...
<thumbnail-url>
http://img.howcast.com/system/thumbnails/180126/
Picture_4_xxlarge_maintained_aspect.png</thumbnail-url>
The thumbnail size for the markers (steps, tips, facts, and warnings)
is 80x45.
<markers>
<marker>
<id>69605</id>
<position>8</position>
<timemarker>24</timemarker>
<type>Step</type>
<thumbnail-url>
http://img.howcast.com/system/thumbnails/
180126/24_medium.jpg</thumbnail-url>
...
</marker>
There can be slight discrepancies (e.g. +/- 1 pixel) as you pointed
out based on the resolution of the video, but for the most part things
should follow this size.
Thanks,
- Russ