Alot of planning goes a long way to the final build , I would decide on what you want out of the kart , then decide on what engine that will give you the needed requirements for power & speed , what type of clutch setup , your final drive ratio , brakes , sit down on the floor as if you were in the kart , measure from your back to about 6"`s in front of your feet , measure how wide you are on the floor for width , add a few inch`s on each side to that , that will give you a good starting point to designing a frame to fit you , then you`d have to add in how much frame is the engine going to need , are you going to use a live axle or single drive wheel ..
A easy way to get a good idea of a frame is to do alot of searching on the net or the forum and look at other karts to get ideas , find one that you like and go from there , a budjet is allways a limiting factor in how far your project will go , good luck with your build , Bill