Instructions
Open "Fallout 3" or "Fallout: New Vegas" as you normally would. Select the save you want to start from once the game loads.
Press your keyboard's tilde (~) key to bring up the console. This key is usually located under "Esc" and to the left of "1."
Type "setgs fjumpheightmin" and press the space bar. Input the number you would like to set your jump height as, pressing "Enter" to enable the command. The default jump height is 64, so entering a lower number reduces jump height, while entering a higher number makes your character jump higher.