How would I size a heating system that would only be used to warm the floors?
Your goals are somewhat at odds.  Even where radiant floor heating is used as the only source of heat, the floor typically does not get anywhere near "toasty."  The floor temp is typically only 5 to 10 degrees above ambient (75 to 80 F), and the vast size of the radiator (i.e. the entire floor) provides enough energy to heat the entire space with such a small differential.  So, the floor will feel better than an unheated floor, but since it is still colder than your body temp, it won't feel "toasty" by any means.  So, if you piped enough energy into your floors to make them feel warm under foot, you'd likely overheat the house!  

The only "toasty" floors I've experienced is a heated bathroom floor.  In that case, there was electric coils under the tile (several hundred watts worth for a large bathroom), and indeed, the floor felt warm underfoot.