Getting thumbnail sizes

When I’m defining image sizes, like say add_image_size('my-slide', 600, 400, true); is it possible to somehow retrieve those numbers at a later point? For instance, if I want to define an image size, but only if it hasn’t been defined already by a plugin?

Obviously the information is stored somewhere, presumably in the DB. I’d like to access it programatically, if that’s possible; if not, I might have to go poking in the DB. (wp_options table, maybe?)

Thanks!