The only thing gained by makign those epoxy LEDs larger (say, going from 3mm to 5mm to 10mm) is that the light is more focused with a larger epoxy lens. However, overall output is the same in every case due to the fact that the emitter is encased in plastic, it is thermally insulated -- this heating is what limits the output. In order to keep from overheating, those kinds of emitters must be driven at very low currents to have any sort of longevity. Many cheap flashlights or brand-x household bulbs that use these plastic LEDs that use them will overdrive them, and lead to lots of ugly premature failures.
Higher-power LEDs all tend to be surface-mountable, and require being mounted to a good heatsink. Even these can be driven to more than 50 times the brightness of the best 5mm LEDs, heat is still a problem for making household light bulb replacements. The problem there is that most light bulb sockets are thermally insulated as well - as they are designed to contain the heat from incandescent lamps, so making an LED lightbulb to fit into a regular socket still presents a problem, both from the perspective of heating, and (more importantly) cost.
Also, screw-in light bulbs are not the best use for LEDs -- the advantage of LEDs is that they can be focused, so IMO the best application for them is for small track lighting applications, or in custom fixtures (not retrofit bulbs). For lighting up an entire room, the best bet is to get a good fluorescent fixture (not the crap sold in most stores) -- the best fluorescenst are cheaper, have better color output, are more efficient, and have longer practical life than any LEDs I know of (the only LEDs that would have longer life in real-world use are single-color inidicator lights).
