Hi Larry,
welcome to the Apple (user-to-user) Community!
Re: Voice Control:
Am no expert in this area, yet your request surely deserves some response!
Fortunately, Apple documents the way the things work in support articles (links below).
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Re: "The hints feature is useful but very small and disappears very quickly.
Any way to make them bigger and stay visible for longer?"
To quickly magnify small wording on the screen, you may want to enable the use of hover text.
When on, any wording under the cursor is immediately shown large by pressing the Command key.
See how to: Use Hover Text on Mac - Apple Support
Eg: says:
"Turn on Hover Text
On your Mac, choose Apple menu > System Settings, click Accessibility in the sidebar,
then click Zoom on the right. (You may need to scroll down.)
Turn on Hover Text".
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Re: "visible for longer" Suggested commands are displayed only if the option “Show hints” is selected in Voice Control settings. I don't know of a way to make hints stay on screen longer: "You can turn standard commands on or off and create your own commands, delete them, and import or export custom commands." See how to: Customize Voice Control on Mac - Apple Support
or
"To see a list of the Voice Control commands you can use with VoiceOver,
choose Apple menu > System Settings, click Accessibility in the sidebar,
then click Voice Control on the right. (You may need to scroll down.)
Click Commands, then scroll to the set of Accessibility commands".
More info: Use Voice Control commands to interact with your Mac - Apple Support
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Re: "There also seems to be a problem with feedback between Typing Feedback to Voice Control.
That is the Typing Feedback is picked up by Voice Control - can this be stopped?"
To turn off typing feedback :
Apple Menu > System Settings > Accessibility > Spoken Content > Typing Feedback
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Re: "Just trying to get the hang of Voice Control".
If you haven't discovered it yet: Apple has a useful resource for getting to know Voice Control:
Voice Control, Switch Control, and Accessibility Keyboard User Guide for Mac - Apple Support
All the best :-)