They have the same base sizes as the old bulbs. Small, medium, & large. Medium is what fits most lamps. You don'! Need a special fixture. So far I've changed 2 lamps to led bulbs. One is a three way, the other is a desk lamp.
That inspires another question. The LEDs I've seen look like they are directional, kind of like a floodlight. Do they actually provide an area lighting like an incandescent or CFL?