@YULIA-2016 I asked the same question to tweeter "I am just wondering should family size counted by years? and should the person deceased in 2018 included in 2016 and 2017 family size and not in 2018 family size? what about decreasing family size, there is no any example. Thank you!" and I got this reply "Hi. Please include the deceased family members in family size for any year that they were alive." It is strange for me too, but...
I really confused by this too, as in my case it is divorce and they are advising to include my ex spouse in 2016, as I was separated in 2016 and divorced from 2017. But try to refer to Guide that according to Guide there family size should be calculated based on current family size and there are only examples for increasing family size, and no any example for decreasing. For prior years the family size was calculated based on current family size, the guide was not changed, but it seems the interpretation was changed