“had two edges.” The Hebrew is literally, “two mouths.” The sword “ate” in both directions. The weapon was actually a long dagger, and the fact that it had two edges is important because a double-edged dagger can more easily be thrust straight ahead into a body. While stabbing can be done with a single-edge blade, a double-edged weapon cuts both sides of the wound (and can be wiggled side to side for even more damage) and thus causes much more bleeding than a single-edged weapon. If the dagger pierces the heart, the issue is moot, but Ehud had no guarantee that he would land a blow that killed quickly—many stabbing victims die because they bleed out, and so Ehud gave himself the best advantage he could when selecting the weapon.
“gomed.” A “gomed” is likely just shorter than a cubit. Likely 16 inches or so.
“he strapped it under his clothing on his right thigh.” The fact that Ehud was left-handed may have really helped him here. It is quite likely that Eglon’s guards would have thought that Ehud was right-handed and thus would not have looked very hard at his right side, but would look more closely at his left side to see if there were any signs of a weapon.