Try this css snippet (adjust the 1em up/down to your preference).
.markdown-preview-view code {
font-size: 1em;
}
The downside is it seems to change the inline code block size as well (which is larger by default than a code block). Anyone know how to adjust only the inline code styling? I was using this css (size + black/yellow styling) which worked fine until a recent Minimal Theme update. The second bit required editing the theme css itself for some reason.
.markdown-source-view.mod-cm6 .cm-inline-code {
color:rgb(216, 186, 118) !important;
background-color:rgb(46, 50, 53) !important;
padding:0em 0em;
}
.cm-s-obsidian span.cm-inline-code {
color: rgb(216, 186, 118);
background-color: rgb(46, 50, 53);
font-size: 0.92em;
padding:0.15em 0.15em;
}