I just submitted our final numbers to the venue, and they made a comment about how it was smart that we had reserved the bigger one of their private dining areas after all. It made me remember that when we first started planning, we thought two friends and their partners, plus my FH's grandparents were likely to decline and so we considered the smaller room. In the end, we decided not take the chance and got the big one.
Now we are getting married in two weeks, we have 100% yes RSVPs and the venue says both rooms are booked solid for months. So if we had got the smaller one, there would have been no way to change it now - we would have been hugely screwed. I'm really glad we played it safe.
So for couples counting on declines to fit into their venues, please reconsider! Sometimes the people you don't think will make it move Heaven and Earth to get there. Be extra safe with this.