I can't say that there is a generic set of numbers to work with... When I lay out the design work on a new rig, there are tons of stuff I'm keeping in mind.

The quick and dirty version..

1.) I lay out the suspension geometry first and map out the range of travel up and down...and draw in the tire size. This lets you see what kind of clearance you need.

2.) Look at the length of your shocks at your intended ride height. take a compass and locate where the shock will be located on the axle and draw an arc for the range that the top of the shock will be at ride height for the front and rear shocks..

This will give you some constraints to try to keep your tube work inside of... You know that tubing has to hit the arcs to mount the shocks and you know that the base of the chassis has to hit the control arm locations as well as clear the links..

3.) carefully measure your engine combo and draw it in like a block... this will let you see where the top of the trans will be, the top of the engine and oil pan... how far foreword the accessories set, etc. Make a template to move around on the drawing if you have too. determine where the bellhousing will be in the chassis.

4.) get the actual seat you plan on using and set down in it. get someone to measure how tall you are from the ground for head height, how far your arms reach out when your comfortable, where your feet are when they are comfortable where your elbow is when your holding a steering wheel... plot these measurements in the drawing and it will tell you where to put the seat, how high the beltline of the chassis needs to be to be comfortable and how high the roof needs to be to be safe...

5.) Start measuring each component and place where it will go in the chassis... Think ahead of EVERYTHING, then go look for parts that fit the space or design the space to fit the parts. I'd strongly encourage looking at the generic parts that are available and design the chassis to fit them... this will save lots of money in your build, custom parts get expensive quick!!

All these plotted points will start to let you know where the main structure of the chassis needs to be to hit critical suspension points and fit components while saving enough room to be comfortable. From there you have to play with component sizes and the lines in the chassis to make it look good...

Even knowing all the "critical dimensions" doesn't ensure the rig will have style. Sometimes changing the angle of even a single tube just a few degrees can have a HUGE impact on the attitude and character of a chassis... The main thing is to spend a lot of time drafting out your ideas and don't even think about trying to build until your happy with it on paper and can see that things will fit where you intended.