I’m not sure if you could make a fission reactor that small. According to Wikipedia the critical mass of americium (the stuff in smoke detectors) is between 57 and 75 kg, so that’s more “you could put it in a car” size. I don’t really know how critical mass relates to functional fission reactor designs, though.
Decay is said to produce about 115 W per kg, so you could make a radiothermal generator, but I expect that once you’ve factored in the other required parts it would still stretch the definition of portable, even if you’re only looking to get enough power out for a phone.
EDIT: yeah, see:
Prototype designs of Am RTGs expect 2–2.2 We/kg for 5–50 We RTGs design, putting Am RTGs at parity with Pu RTGs within that power range, as the vast majority of the mass of an RTG is not the isotopes, but the thermoelectrics, radiators, and isotope containment mass.
