A similar issue can be also observed in RadTreeView:

Workaround RadGridView: 
private void RadGridView1_CellEditorInitialized(object sender, GridViewCellEventArgs e)
{
    RadTextBoxEditor editor = e.ActiveEditor as RadTextBoxEditor;
    if (editor != null)
    {
        editor.EditorElement.MinSize = new Size(0, 30);
    }
}

Workaround RadTreeView:
 
private void RadTreeView1_EditorInitialized(object sender, TreeNodeEditorInitializedEventArgs e)
{
    TreeViewTextBoxEditor editor = e.Editor as TreeViewTextBoxEditor;
    if (editor != null)
    {
        ((BaseTextBoxEditorElement)editor.EditorElement).TextBoxItem.TextBoxControl.MinimumSize = new Size(0, 24);
    }
}

In 125% the RadTextBox in the Fluent theme keeps shrinking:

Workaround
public RadForm1()
{
    InitializeComponent();

    SizeF dpi = NativeMethods.GetMonitorDpi(Screen.PrimaryScreen, NativeMethods.DpiType.Effective);
    this.radTextBox1.MinimumSize = new Size(0, (int)(24 * dpi.Height));
}