You may adjust font-size, like this:
div.da-reactions-container div.reactions div.reaction .count { font-size: 0.3em; }
Using the complete hierarchy in selector give the right priority, but event this will have the same effect:
.count { font-size: 0.3em !important; }
It is important to avoid the !important keyword because it could bring to unexpected results, moreover a complete hierarchical selector is good to avoid collision with other elements in page.
With this selector you may change everything, for example you may display numbers under images:
div.da-reactions-container div.reactions div.reaction .count { position: relative; right: 0; line-height: 1em; background: transparent; }
You can probably add additional CSS directly through WordPress interface if your Theme supports this feature, it should be at Appearance » Customize » Additional CSS.
If your theme does not support this feature you can always write those CSS lines in your main CSS file.