I was able to find a work-around. When I set the secondary taskbar to Align Right, I can resize it. Once I have the width I like, I can set it back to Align Left, and it stays locked at the new width.
Odd thing: Even the primary taskbar, when set to Align Left, is locked width. Putting it at Align Right lets me resize it again.
Not sure what can cause this behavior. I am running Stardock Fences5 and Stardock WinBlinds Service.
Explorer Patcher was uninstalled when I installed Start11 v2.5, as "enhance the taskbar" was unavailable - a line under the setting stated there was a conflict. I find that Start11's version of the vertical taskbars a bit better than Explorer Patcher's.
Thank you for the vertical taskbars that look nice and work well. I will eagerly wait for any updates and patches that provide the features I was mentioning.