is it possible to specify the sizes of images with yahoo images api?
Here is the docs. Nothing about this future. And it's available in web interface.
No, the original Yahoo Image Search API did not allow specifying the image size. (That service was also deprecated as of April 2011, so it's not much use now.)
The Bing Search APIs do allow for a sized image search. Look for the "filter" options in the request, with sourcetype set to images.