All I can make out from that is something about the mind eye (心眼) becoming ever so slightly widened (about fingernail size)
The expression "心眼小" means petty, and is usually applied to money matters. So by saying that the "心眼" is as small as a fingernail is to figuratively say that the person in question is miserly and not generous with their money. By the way "心眼" by itself can mean all sorts of different things so most such phrases need more context to reveal what the meaning is.
Was just about to say the same, Give Context! It's so hard without it. I can guess different things. Also, it lacks a preposition, which is normal for Chinese, but you might need it in the translation. It sounds awkward to say 'The conscience is as big as a fingernail!' Should be 'His/Her conscience'. Other than conscience, it could be patience? I can think of a few other fitting words. Not sure. I need the situation that led to this statement. Closest to English I can come up with :"He/She's so narrow-minded!" or "so cheap!" Again, without context I have no idea which is right.