I think this is actually something missing from Ash.Filter, specifically when given a nested map like that we should check for embedded resources and allow you to apply that kind of filter. Could you open an issue for it in Ash?
Another way to achieve this kind of thing is to add calculations to the root resource, i.e
```elixir
calculate :size, :string, expr(data[:size])
and then allow filtering on :size. Not a feasible option for every case though.
