Hi!
This seems to be a bug with the new workspaces-interface design and can easily be reproduced:
1. Just open Maya with Maya Classic Workspace
2. toggle from Channel Box to Attribute Editor
3a) switch Workspace to any other
4a) switch back to Workspace "Maya Classic"
or
3b) close Maya
4b) re-open Maya
...now toggle with ctrl+a.
Workaround:
To restore Channel Box-Size do it the other way around.
1. In Current Workspace ("Maya Classic") toggle to Channel Box
2. Resize Channel Box.
3a) switch Workspace to any other
4a) switch back to Workspace "Maya Classic"
or
3b) close Maya
4b) re-open Maya
done.
So, if the Channel Box is open before you close Maya or switch Workspace everything is fine,
but not so with Attribute Editor open.
I know this is not a final solution, but maybe helps for the moment.