I'm trying to change the size of the images of the upsell products shown on the product page, but can't figure out how to do it. I've got the Ultimo theme.
You can adjust the size multiple ways and i would start by look at..
and find the line (around line 38) and adjust the resize value, width & height
<img src="<?php echo $this->helper('catalog/image')->init($_link, 'small_image')->resize(125) ?>" width="125" height="125" alt="<?php echo $this->escapeHtml($_link->getName()) ?>" />
you may also need to change via css if the style is controlled by that.