Windows Form Application "How do I increase the size of the button when the mouse hovers over the button?"

Use the MouseEnter event to capture the mouse cursor hovering over the button borders,
and the MouseLeave event to detect when the cursor leaves the button borders, in order to return it to its original size.

Here is one way to implement such functionality:

private Size OriginalButtonSize;
private Size HoveredSize;
private void Form1_Load(object sender, EventArgs e)
{
    OriginalButtonSize = this.button1.Size;
    HoveredSize = new Size(OriginalButtonSize.Width + 30, OriginalButtonSize.Height + 30);
}

private void button1_MouseEnter(object sender, EventArgs e)
{
    button1.Size = HoveredSize;
}

private void button1_MouseLeave(object sender, EventArgs e)
{
    button1.Size = OriginalButtonSize;
}

Output: