using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;

namespace WpfApp1
{
    class ByteLimitTextBox : TextBox
    {
        public static readonly DependencyProperty MacByteProperty =
            DependencyProperty.Register("MaxByte", typeof(int), typeof(ByteLimitTextBox), new PropertyMetadata(null));

        /// <summary>
        /// 最大バイト数
        /// </summary>
        public int MaxByte
        {
            get
            {
                return (int)GetValue(MacByteProperty);
            }
            set
            {
                SetValue(MacByteProperty, value);
            }
        }

        protected override void OnPreviewTextInput(TextCompositionEventArgs e)
        {
            base.OnPreviewTextInput(e);
            Encoding en = Encoding.GetEncoding("shift-jis");
            //入力された文字列のバイト数取得
            var byteCount = en.GetByteCount(this.Text);
            if (byteCount >= MaxByte)
            {
                string newTrxt = new string(this.Text.TakeWhile((c, i) => en.GetByteCount(Text.Substring(0, i + 1)) <= MaxByte).ToArray());
                this.Text = newTrxt;
                this.Select(this.Text.Length, 0);
                e.Handled = true;
                return;
            }
        }
    }
}

<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="400">
    <Grid>
        <StackPanel VerticalAlignment="Center" HorizontalAlignment="Center">
            <local:ByteLimitTextBox MaxByte="14" Width="150"/>
            <local:ByteLimitTextBox MaxByte="14" Width="150" Margin="0,20"/>
        </StackPanel>
    </Grid>
</Window>