In this link you can find css for obsidian slide, e.g. increase font size (to fit all content) or heading color etc. but it doesn’t have any css for code block.
so, you can use below css code to increase the font size and width of code block for the presentation slide in obsidian.
just copy this codes to a file (e.g. MD_slides_obsidian_snippet.css) and move that css file to your .obsidian\snippets folder. Then you can turn on this snippet from the appearance setting.
.reveal pre {
font-size: 0.9em; /* increase font size */
margin-left: -5%; /* decrease left margin */
width: 110%; /* increase width */
}