Use attached project to reproduce.

Workaround:
foreach (RadPanel item in radDataEntry1.Controls[0].Controls)
{
    var textBox = item.Controls[0] as RadTextBox;
    if (textBox != null)
    {
        textBox.TextBoxElement.MaxSize = new Size(0, 20);
    }
}

or
this.radDataEntry1.ItemDefaultSize = new Size(200, 24);