I think it's easier to make an ADR for a party of two. This is because I can go in and select any size party (if there isn't one available for 2, I can take one for 5). Then it's easy to go into the reservation, select modify, and adjust the size down to 2. So pretty much any size party (except 1, obviously) will work for getting an ADR. If there's nothing available for 2, especially for our preferred time, I automatically increase the size of the party and keep looking.
I feel like this is an advantage over times when I need an ADR for my entire family (7), and there's zero availability for larger parties, but when I've looked out of curiosity, I've found ADRs for smaller parties (sometimes I can break us up into smaller groups, and sometimes I can't).