Since the axis travel distances of my machine was based on what size of linear ways could be won on eBay auctions... I had no firm grasp of what I would eventually end up with for a machine size. I had an idea of what I wanted... but the fiscal realities at times forced a compromise or two which usually resulted in a down sizing of my machine. Those longer linear ways were typically too expensive for my budget.
Once I had the X,Y, & Z linear ways in hand... I was able to finalize the dimensions of my machine. The machine was drawn based upon the dimensions of the THK & Hiwin ways I won on eBay. The way I went about determining my cutting area or as you put it... the size of the cutting table... was to draw my machine in scale, then using the drawn components placed at max travel, you can locate the max cut of the tool bit. I did this for the Y-axis and X-axis. Knowing the dimensions of your components is a must so you can produce accurate drawings or CAD plans. The attached drawings might shed a bit more light on this sizing of the cutting area... and in the 2nd drawing the orange rectangle represents my cutting area based on the max travel distances of the X & Y axis.
As far as the ballscrew dimensions... I knew going in that I wanted to use DumpsterCNC products... and selected his 1/2" 5-start ABN, clamps, and coupler's because of their reputation, cost and availability. The 5-start was chosen for speed.
Hope this helps...