You can increase/decrease the font size in the chat window when there are closed captions. From the CC icon, CC > Subtitle Settings > Accessibility > Chat Display Size. 
If there aren't closed captions, go to your video settings (use the caret next to the video camera icon) and then Accessibility.
When I started a Zoom meeting, I was able to use the Control +/- as a host. I wasn't able to do so as a co-host or participant. I am using a PC desktop with W10.
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Alan Katsura
Moderator HLAA Tech Group