Welcome to the Router Forums David. You really need to decide the working size you want and then base screw sizes base on that working footprint. Of course, the screws will be longer depending on which axis you are building but you can make sure you get what you want in you CNC.
I think the important thing to remember is don't limit yourself in size because you can get frustrated with a machine that is too small and then you will want that bigger machine. Another thing to remember most of the small entry CNC machines and DIY CNC machines have little or no resale value so it could be wasted money if you don't build the machine you really want.
Your BRAIN Is The Most Important Power Tool In Your Shop. Turn It On Before You Turn On Any Other Power Tool.
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