In my application I have a number of toolbars. How to create one of the toolbars with a different image/button size?

By design, all buttons in the customizable toolbars should have the same image size. Otherwise, it will be impossible to move a button from one toolbar to another. However, if you need a toolbar with either larger/smaller images, you need to create it as "locked" (non customizable). To do it, just set bLocked parameter of CBCGPToolBar::LoadToolBar to TRUE:

void CMainFrame::OnCreate (LPCREATESTRUCT lpCreateStruct)
{
....
m_wndToolBar.LoadToolBar (IDR_MAINFRAME);
m_wndToolBar2.LoadToolBar (ID_OF_LARGE_TOOLBAR, 0, 0, TRUE /* bLocked */);
....
}