Just an off the cuff guess, but if you switch it to edit mode it will likely display as a text area. It probably tries to conserve space in detail mode so since you only have one line of text, it only displays as one line.
We don’t have options to show how many lines the editor should be. Raymond is right - in read mode we only show the space actually taken (or the max characters defined in the field setup). In Edit mode, if it is anew record, you will only see one line and the text editor will expand as you continue typing. Once you click into the field (not just get it into edit mode) you will see the Rich Text editor control.
I do not think that is how it is behaving. I have shot a video to show how a TextArea behaves vs the RichText behaves. It would seem the 5000 character field would have the ability to see more than 1 line?