Outgoing + ( 0.5 * Nice ) / Aspiration ACR Rank = Ideal Family Size
This formula fits into my spreadsheet quite nicely without having to collect any additional information such as interests or making the formula complex by considering family background (would those sims from large families prefer large families as well since they enjoyed the experience, or would they prefer smaller families in an attempt to give each child more attention)
Based on primary aspirations only, the formula gave pretty ideal results of what I envision each sim as desiring for their ideal family size. There were a few surprises, such as Jesse (Carr) Kent wanting 14 children since he's a very nice, outgoing family sim. Which leads into the next topic, that just because they have that ideal family size doesn't mean that they will have that many children. Thanks to ACR try for baby based on aspiration and my own birth control rules, then some sims won't have their ideal family size-sorry Jesse. I also noticed that as of now, the average ideal family size for my hood is 3.6 which fits into what the size of most families. I have also included a chart of how the fertile sims (teen and older) in my hood view their ideal family size.
Of course, the actual size of the family is also regulated by finances (can the couple "afford" another child?) , age (older sims have lower fertility rates with ACR), unplanned pregnancies, medical issues (the "barren" trait), spousal preference (for those sims who want 2 children vs 7 children) and of course their wants (if they both roll wants for a child, then I remove birth control, if the partner with the lower child tolerance rolls a want for a baby, then I also remove birth control) .