A radio activated pill about 15mm long by 5mm diameter would probably do the trick.
The human body is fairly transparent to 100MHz radio signals.
The speed of light in human tissues and fluids is roughly 1/6th that of free space.
Therefore the wavelength of a 100MHz signal passing through the human digestive tissues and fluids would be roughly c / 100MHz / 6 = 50cm (as opposed to 300cm in air).
A receiving antenna typically needs to be 1/4th wavelength, which in this case is 12.5 cm.
Construct a pill having an inner enclosure. An antenna made of 12.5cm of 30 gauge enameled spring wire, and a gelatin outer coating.
The antenna spring wire wants to spring into a straight position, but is coiled around the inner chamber and held in place by a gelatin coating. You would need to wrap the antenna 12.5cm / 5mm / pi = 8 times around the body of the 5mm diameter capsule to get the required length.
When the pill is ingested the gelatin dissolves and the spring wire extends to its naturally straight position. The human stomach is about 12 inches long so there is more than enough room for a 12cm antenna to extend.
The body of the inner capsule contains two electrodes. One made of aluminum and the other made of copper oxide (you can pick any other two metals if you wish). When the gelatin outer coating dissolves and the electrodes are exposed to stomach acid they will form an aluminum copper battery that will power the receiver. Prior to being exposed the battery will have no electrolyte and will remain inactive.
The inner chamber is sealed and contains a small low-voltage ASIC chip, a small charge of gunpowder, and the required poison. Upon receiving the signal the chip will use the charge in the battery to ignite the gunpowder thus releasing the poison.
Or You could bluff
Create a poison pill with a coating that dissolves in a certain amount of time. The amount of time it takes the stomach acid to dissolve the coating would be proportional to the thickness of the coating.
Based on your knowledge of how the time release mechanism works you have a good guess of when the victim will fall ill. A little before the appointed time you watch closely for signs that your victim is being affected. Your well trained eye allows you to see the effects before others notice it. Then announce that you are going to trigger the poison. Pull out your remote and conspicuously press the button. Soon afterwards the victim dies. Everyone assumes it was you.