This has nothing to do with being "too big to be a set", but only "too vague, as written, to be any specific set (and therefore any set at all)".
For any sets $U,V$ and $W$ the collection of maps from $U \times V \to W$ is a set. The same is true if the sets have the additional data of vector spaces and the maps are required to be bilinear. "Bilinear maps from $U \times V$" can be construed as a function, or a functor, taking a set $W$ as input and producing the set of bilinear maps into $W$ as an output. This is well-defined and well within the realm of sets. There is also the construction of a universal object $W_0$ as the target of such bilinear maps, and the discussion of whether an unspecified collection of bilinear maps is too big may be just a way of motivating the generators-and-relations construction (which is big, but not too big or too vague to be a set).