I make a small image of a skeleton inside a figure and scale it up to about 4 foot tall. I also arbitrarily decide that the final weight should come in at about 5 kg. Much heavier or taller and I'm going to have difficulty physically handling it during testing, catching mid fall etc. Also taking into account the tools I have. Just the usual hand tools, bench vice and an ancient MDX 20 model making CNC machine.
Spend a week fretting over what to use for actuators. Servos that are able to suppy the torque needed, and maybe survive the inertia of the limbs, are just way way too expensive. Two of them would eat the entire budget. Linear actuators, on the other hand, look much too heavy as do all the direct acting off the shelf motor/gearbox combinations I can find.
Decide to make my own actuators. Rack and pinion seem the simplest, lightest and easiest type to make. Bought some smallish 37mm general purpose motors/gearbox from Pololu , and some nylon racking and pinions, and also some delrin sheet to mill out to hold to parts together.
The MDX 20 is not a fast machine, but over the course of a week it stoically machines 6 gearboxes, as well as a batch of bushes I'll use as pivot bearings and some horns for the pots.