I would like to create an abstraction of neuron.

My pb is that I would like to be able to set a determined size for my vector input (with the help of a message box for instance) ... the t f f f f is too restrictive.

does anyone know a way of doing that ?

http://www.pdpatchrepo.info/hurleur/abs_neuron.pd

will give you the size of a list.
[drip] will unfold a list into a sequence of message, check its help.