Walking robots are notoriously difficult to get working properly. If you’d like a good place to start mechanics-wise, Technovation's 3D-printed biped may be just the thing for you. The bot boasts a six-servo design to control its hips, knees and ankles, on top of which is situated an Arduino and perfboard PCB.
The device’s joints are fastened by the servo horn on one end, but feature a bearing assembly on the other so that they don’t skew under loading. Walking motions are computed using inverse kinematics, allowing the user to simply program in X and Z coordinates, and it will then produce the correct leg angles.
The robot can reportedly take steps of between 2 to 10cm without tipping. If you decided to modify this project, constants for the hip-knee and knee-ankle distance can be input to accommodate for these variations.
An Uno acts as the brains of device, optionally via the perfboard PCB/headers on the back. While it looks nice set up as is, the top portion appears to be quite modular and could easily form the basis for whatever controls, sensors, and other accessories you have in mind. Code and print files are available in Technovation's post.