In my Linux environment, Logseq 0.8.5 binary installation, I use (Ctr…Shift…=) for Zoom-In and Ctr…- for Zoom-Out.
I also tried changing the font size permanently by changing the custom.css of the graph. This is what worked for me
:root {
font-size: 18px !important;
}
a.tag[data-ref] {
font-size: 18px !important;
}