I've been unable to find anything that is definitively less than 20 people, but there are a few promising options.
Day trips between Boston and New York typically leave around 7 a.m. and bring you back to your starting city around anywhere between 7 p.m. and 11 p.m. (that's scheduled only - do expect delays due to traffic and construction, as the entire route is extremely heavily traveled). About $150-160 per person (including children) seems to be the going rate for transporation + guide, though you'll have to pay food and possibly some admission fees on your own.
TakeTours has 1-day trips from NYC to Boston for about $149 per person. These tours are available year-round, are fully-guided, and provide transportation in either a bus or van. I couldn't find any information about the typical size of groups; they clearly do small groups since one of the transportation options is a van, but I don't know if they cap tour numbers or just decide the vehicle based on how many spots are booked.
CitySightsNY has day trips that are about 12 hours long, but they are only offered on Fridays and Saturdays. These are also $149 per person (child and adult). The majority of their services are tours within New York City, and that's what all of the reviews I found were for. Those tours feature large, "hop on, hop off" buses or serve large groups. I haven't been able to find any information about what size groups they take for day trips, but I can tell you that after growing up in the Boston area, currently living in NYC, and frequently traveling between the two, I suspect the day trips are much smaller groups - I've never seen CitySights' distinctive huge blue buses outside of the city.
Tours4Fun offers day trips from NYC to Boston on Fridays and Sundays for $149+ per person. Also unclear what the typical tour group size is, but they mention luggage restrictions for tours by van, so you may be able to request a smaller group.
Honestly, if you're willing to spend $200-250 per person, you're really better off booking your transportation to and from Boston separately from a tour. Bus is by far the cheapest way to get between the two cities (Megabus, Bolt Bus, Peter Pan/Greyhound, and Fung Wah tend to run $20-40 per person round trip), and you'll have a LOT of choices for when you leave and come back. Just make sure you pay attention to whether you're booking a reserved seat (good only for your selected route/time) or if you're booking a ticket that can be used for any departure that day. All of those companies will pick you up and drop you off at convenient, central locations.
Once you're in Boston, you can tour at your own pace. Many historic sites that tours would take you on are free (like Quincy Market/Faneuil Hall, Harvard Square, Boston Common, etc.), but if you're looking for some history you can purchase audio guides. (The Freedom Trail has an audio guide you purchase online for $15 and download to an mp3 player in advance, for instance, and you can do the Boston Harborwalk with a free audio guide.)
My recommendation would be to book your own transportation at times convenient for you, take a look at the itineraries of some of the tour companies, and build your own custom tour. It's the only thing that really makes sense if you don't want to be slowed down by a large group but want to get some more in-depth knowledge than you'd get just by wandering around reading the (sometimes sparse) signage.