A real big flower, 4.5" or more, is more than likely a pure vanda. Ascocendas usually are smaller, but some may be larger depending on how much vanda is in its genes. I've never seen an asco with flowers much more than 3.5 or 4", however.

There are also pure vandas that produce small flowers, although they have not been used that much in breeding, except to impart some "nonstandard" colors. But to me, the name "vanda" demands a big, in-your-face dinnerplate with bright gaudy colors. The small stuff just doesn't cut it for me.