How can I make a button to be in pressed state in C#?

It works!
I used a CheckBox with the Appearance property set to Button and it's exactly what I wanted.
But why the size of the button is so small?
I can not change the height and the weight of the button? Why?

What is that you want to achieve using this? If you want to toggle something there can be ways to do it.

If it's about showing the button pressed... you can do that too using css or javascript. Something like, Click once button control, it will be pressed as long as response is not recieved. Now in your case, what is the logic of keeping it pressed?

And what you want to have on the "button"? I used it in toolStripMenu. If I set size, no change. But when I set size and image on the button, it has size, which I want and shows full image.

CheckBox bold = new CheckBox();
bold.Appearance = Appearance.Button;
bold.Size = new Size(16,16);
bold.Image = Properties.Resources.b;

Where image from Resources has size 16x16px.