[michaf]

I am not an expert, but maybe look into recurrent neural networks [0]? These things can turn a sequence of inputs of possibly arbitrary length into a fixed size internal representation, and can output a single value derived from this representation.

[0] https://en.wikipedia.org/wiki/Recurrent_neural_network

[anon1253]

most implementations are not arbitrary length, but instead rely on padding to make it a fix length sequence.