I don’t know, they’re already pretty small.
It’s a pretty fair question though, since most of us remember when TV antennas were over 6 feet long even if you lived in a suburb fairly near a city. The only way you could get away with a smaller antenna was if you lived right in the city itself. Then, about 20 years ago we started seeing compact antennas that had just as much power as larger ones. Is it too much to hope that we’ll see another revolution in antenna size?
Unfortunately it’s not likely to happen. Two factors combined to make antennas smaller, and unless something really impactful happens, you won’t see a revolution like that again.
Amplifiers got really good.
One of the easiest ways to get more signal from an antenna is to amplify it. This doesn’t work perfectly, since (1) you need to actually receive a signal in order to amplify it and (2) since amplifiers can add noise, you need to be able to amplify enough to overcome that noise. The amount of noise you add is especially important with digital signals.
What we’ve seen in the last couple of decades is inexpensive low-noise amplifiers that work well with antennas. This has made it possible to build amplifiers right into the antenna so that there wasn’t really any cable or connector loss, and the amplifier could get the maximum signal to begin with, meaning that it could find more channels to amplify.
I’m not actually a huge fan of amplified antennas except in fringe areas where the signal could get swallowed up by noise if goes over a long cable, but I certainly admit they have their place and they have the potential to give you a better antenna with a smaller footprint.
UHF has ruled the day for most of the last decade.
This is the real reason antennas got smaller. As frequencies go up, wavelengths get shorter, and since the size of an antenna is tied to the size of the wave you want to receive with it, a UHF-only antenna is going to be smaller than a UHF/VHF antenna. Since 2009, there have been very few channels in the VHF band and practically none in the VHF-Low band (channels 2-6) where an antenna needs to be well over 6′ wide to do its best work. This means antennas that could be smaller and still do the same job.
If you were really going to make antennas smaller, you’d need to move all TV broadcasting to higher frequencies like the ones used for cell phones. Cell phones have tiny antennas because they pick up higher frequencies, and antennas used for satellite are really tiny — the actual receiving part is behind the white plastic on the front. The rest is just a way to focus the signal.
So, when you look at those two factors, it’s not likely you’ll see another revolution in antenna size. Amplifiers are already really really good, especially for the price, and TV broadcast frequencies aren’t going to change. Unfortunately that means your hopes for a postage-stamp-sized TV antenna that picks up channels from 100 miles are probably going to stay unfulfilled.