Hi @SiriusRex,
Rich text editor has a predefined height set in OOB CSS via height and padding properties.
Now per your use case to reduce it to one line height or for a look alike of text field, you can write CSS overwriting the same.
To target specifically without affecting all richtext resource in general, within dialog content markup we have an hidden field capturing sling:resourceType which will have the value as component path of respective dialog. (screenshot #1)
Using this as an identifier, am suppressing the related properties as shown below
Sample snippet for reference (You can add in project specific clientlibs folder or clientlibs with category - cq.authoring.dialog as this is needed only in author)
Below is per AEM 6.5.0
$(document).on("dialog-ready", function() {
var rescType = $(".coral3-Dialog-content input[name='./sling:resourceType']").val();
console.log("Resource Type="+rescType)
if(rescType.includes("learnings/components/content")){ // applies to all component dialogs under this path - learnings/components/content
$(".cq-Dialog:not([fullscreen]) .cq-RichText-editable").css("height", "unset");
$(".cq-RichText-editable").css({"min-height":"unset", "padding":"unset"});
$(".cq-Dialog[fullscreen] .cq-RichText .rte-sourceEditor").css("height", "unset"); // for full screen
}
});
Screenshot 1:

Outcome in richtext field:
