I take it this is a WinForms app. The window will have a Resize event which fires when the control gets resized (funnily enough). You could use this event to check the size of the form, and if it is greater than the max size you want just force it to the correct size . Something like this(pseudo C#)
void Resize(object sender, eventArgs e)
{
if (this.Width >= MaxWidth)
{
this.Width = MaxWidth;
}
}
Hope this helps
[UPDATE]
Here is a VB version
Private Sub Form1_Resize(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Resize
Dim maxWidth As Integer = 800
If Me.Width >= maxWidth Then
Me.Width = maxWidth
End If
End Sub
Good Luck