Todd,
You could use another machine as a quide as Tweakie replied. To do the sizing of the members one needs to look at the total gantry and how it will be framed and anchored. The gantry top frame should be sized to address static loading,deflection, running loads, and also address vibration. The analysis is somewhat subjective since mass and desired rigidity come into play. What one accepts as adequate would be not be acceptable for another. For any machine additional mass is desirable. Since you are buiding the machine you are not restricted to cost restraints and can provide for an improved design.
So you would say that i have not answered the basic question and that is true. There is more to this than just the one member and info you have supplied.
RICH