You can create a file
conf/userstyle.css where you can reset the css for different elements. For instance:
pre {
font-size: 1.25em;
}
This will increase the font size in code and file blocks by 25%, since the default is 1em (dokuwiki template). You should automatically get horizontal scrollbars if the block exceeds the window's horizontal dimension, but you can get vertical scrollbars by setting the height of blocks to a fixed size. For instance:
pre {
font-size: 1.25em;
height: 400px;
}
If you are not using the dokuwiki template then you will have to check the css for your template.