source: Common/DpiScaling.cs

using System.Windows.Forms;
using System.Drawing;
using System.ComponentModel;
using Microsoft.Win32;

namespace Common
{
    class DpiScaling
    {
        public static float SystemDpi;
        public static bool UseCustomDpi;
        public static float CustomDpi = 96;
        private static bool _changed;        

        public static void ApplyToComponent(Component component)
        {
            if (component is Control)
            {
                foreach (Control child in (component as Control).Controls)
                {
                    ApplyToComponent(child);
                }
            }
            if (component is ToolStrip)
            {
                //(component as ToolStrip).AutoSize = false;
                Size newSize = new Size((int)((component as ToolStrip).ImageScalingSize.Width * CustomDpi / 96F),
                    (int)((component as ToolStrip).ImageScalingSize.Height * CustomDpi / 96F));
/*                foreach (ToolStripItem item in (component as ToolStrip).Items)
                {
                    item.AutoSize = false;

                    if (item.Image != null)
                    {
                        Bitmap b = new Bitmap(newSize.Width, newSize.Height);
                        Graphics g = Graphics.FromImage((Image)b);
                        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                        g.DrawImage(item.Image, 0, 0, newSize.Width, newSize.Height);
                        g.Dispose();
                        Image myResizedImg = (Image)b;
                        item.Image = myResizedImg;
                    }
                }
*/              (component as ToolStrip).ImageScalingSize = newSize;

                /*(component as ToolStrip).Font = new System.Drawing.Font((component as ToolStrip).Font.FontFamily, 8.25F * customDpi / 96F,
                  (component as ToolStrip).Font.Style, System.Drawing.GraphicsUnit.Point, (component as ToolStrip).Font.GdiCharSet);
                foreach (ToolStripItem item in (component as ToolStrip).Items)
                {
                    ApplyToComponent(item);
                }
                */
            }
            if (component is MenuStrip)
            {
                (component as MenuStrip).Font = new Font((component as MenuStrip).Font.FontFamily, 8.25F * CustomDpi / 96F,
                  (component as MenuStrip).Font.Style, GraphicsUnit.Point, (component as MenuStrip).Font.GdiCharSet);
            }
            if (component is TabControl)
            {
                foreach (TabPage tabPage in (component as TabControl).TabPages)
                {
                    ApplyToComponent(tabPage);
                }
            }
            if (component is ToolStripItem)
            {
                (component as ToolStripItem).Font = new Font((component as ToolStripItem).Font.FontFamily, 8.25F * CustomDpi / 96F,
                  (component as ToolStripItem).Font.Style, GraphicsUnit.Point, (component as ToolStripItem).Font.GdiCharSet);
            }
        }

        public static void Apply(Form form)
        {
            if (UseCustomDpi) _changed = true;
            if (SystemDpi == 0)
            {
                Graphics graphics = form.CreateGraphics();
                SystemDpi = graphics.DpiX;
            }
            if (UseCustomDpi || _changed)
            {
                form.Font = new Font(form.Font.FontFamily, 8.25F * CustomDpi / 96F,
                    form.Font.Style, GraphicsUnit.Point, form.Font.GdiCharSet);
                ApplyToComponent(form);
            }            
        }

        public static float Scale(float size, float baseDpi = 96F)
        {
            if (UseCustomDpi)
            {
                size = size * CustomDpi / baseDpi;
            }
            else
            {
                size = size * SystemDpi / baseDpi;
            }

            return size;
        }

        public static void LoadFromRegistry(string regSubKey = "")
        {
            if (Application.UserAppDataRegistry == null) return;
            RegistryKey regKey = Application.UserAppDataRegistry.OpenSubKey(regSubKey);
            if (regKey == null) return;

            CustomDpi = regKey.GetValueInt("CustomDpiEnabled", (int)CustomDpi);
            UseCustomDpi = regKey.GetValueBool("StackTrace", UseCustomDpi);
        }

        public static void SaveToRegistry(string regSubKey = "")
        {
            if (Application.UserAppDataRegistry == null) return;
            RegistryKey regKey = Application.UserAppDataRegistry.OpenSubKey(regSubKey, true);
            if (regKey == null) regKey = Application.UserAppDataRegistry.CreateSubKey(regSubKey);

            regKey.SetValueInt("CustomDpiEnabled", (int)CustomDpi);
            regKey.SetValueBool("StackTrace", UseCustomDpi);
        }
    }
}