Palo Alto homeowners who want to downsize face two challenges: finding a smaller home in the city and avoiding a steep increase in property taxes when they sell. In the past 12 months ending Sept. 30, about 414 single-family homes in Palo Alto sold through the regional multiple listing service, according to closed-sale records from MLSListings. The Census Bureau estimates that the city has about 16,800 detached houses. That means roughly one out…