The Ultimate Weapon was designed and later build by Hactar and would combine the core of every star in the universe to destroy it.
The weapon was first commissioned by the Silastic Armourfiends, and was to destroy the whole Universe. Hactar was first shocked by the thought, and designed the weapon to fail, leading to him being pulverized by the Armourfiends, though he survived as space dust.
Later, after manipulating the denizens of the planet Krikkit, Hactar managed to slip the Ultimate Weapon into Arthur Dent's satchel, and Arthur later (while revisiting Lord's Cricket Ground) threw the weapon at a Krikkit robot, which was to activate the weapon with the multi-functional bat. However, Hactar did not put into calculation that Arthur was not athletic at all, and missed the robot by several feet.
Based on this, we know the Ultimate Weapon was about the same weight, size and shape as a cricket ball.