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 or smaller images, you need to create it as "locked" (non-customizable). To do it, just set the 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 */);
    ....
}