Q:

How do i change the pixel size when using SetPixel on a bitmap?

newbitmap.SetPixel((int)clouds1[x].X, (int)clouds1[x].Y, Color.Red);

I set pixels in to a new bitmap with color red.
In form1 i show the newbitmap in pictureBox3:
pictureBox3.Image = CloudEnteringAlert.newbitmap;

But the pixels are very small almost cant be see on the pictureBox3.
How can i make the pixels bigger ?

A:

You can't, but you can draw a circle instead.
using(var g = Graphics.FromImage(newbitmap)) {
    g.FillEllipse(Brushes.Red, (int)clouds1[x].X - radius, 
                               (int)clouds1[x].Y - radius, 2 * radius, 2 * radius);
}

I created an extension method that simplifies drawing a circle (here with float coordinates):
public static void FillCircle(this Graphics g, Brush brush,
                              PointF center, float radius)
{
    g.FillEllipse(brush, center.X - radius, center.Y - radius,
                         radius + radius, radius + radius);
}

If your clouds are PoinF's you can call it like this:
using(var g = Graphics.FromImage(newbitmap)) {
    g.FillCircle(Brushes.Red, clouds1[x], radius);
}

If not, adapt the extension method accordingly. Place such extension methods in a static class.
In order to get smooth circles you can use antialiasing. Set the SmoothingMode of the Graphics object to the desired value before drawing circles:
g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;

