Looks like a nice project
What I found was the best way to work out the size of servos required was to get the torque needed to move the axis from a standstill. That is easy enough to do, just mount a 2m flat bar to the ballscrew at the centre of the bar, then use weights or a spring balance to measure the weight required to start the axis, you can then work out the Nm required. Get a motor with a continuous torque of that or slightly more and you should be fine.
Reason I mount the bar by the centre is so the weight of the bar is balanced thus not giving false readings and obviously you could use a shorter bar and just adjust the calculations accordingly.
Hood