<style>
    span.k-textarea.form-control {
        border: 0;
        box-shadow: none;
        width: 100%;
        height: auto;
    }

        span.k-textarea.form-control textarea {
            border: 1px solid rgb(206, 212, 218) !important;
            border-radius: 0.25rem !important;
           
        }
</style>

<TelerikTextArea @bind-Value="@theValue" Class="form-control" AutoSize="true" />

@*<textarea class="form-control" @bind="theValue"></textarea>*@

@code{
    string theValue { get; set; } = "one\ntwo\nthree\nfour";
}