So baisically your guideline is the main frequency component of the HRF function (which for the "canonic" HRF used in humans just so happens to be 16s), yes?
So ideally, if I determine my hemodinamic impulse response function via a FIR set in pilot experiment, a better estimate would be the main frequency component of that, right?
And I guess the way you address the upper bounds of block length is by saying "well, we want many block repetitions, and a highpass filter as stringent as possible, so we'll just go with the shortest feasible blocks". This argument sounds simple enough, but it is very qualitative, wouldn't you say? Certainly there are benefits associated with longer blocks - such as making sure that the signal returns to baseline.
For instance with a 16s block and a 20s HRF (with the main frequency component of 16s), if you would stimulate for 8s you wouldn't expect the signal to retorn to baseline before the next block starts, no? you would have to wait at least 20s after the end of the 8s stimulation block, no?