Please refer to Ionic documentation for this or any future questions regarding the natural behavior of ionic.
To answer your question: try using autoGrow = "true" as follows :
<ion-col size-md = "12">
<ion-textarea class = "italic" rows="10" cols="5" autoGrow="true" maxlength = "200" placeholder="Please provide additional comments here..." [(ngModel)]="postFeedback.explanation"></ion-textarea>
</ion-col>
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