Belt reduction questions, Please help!?

Looking for some help. I'm going to be converting a X-3 Desktop mill
to Cnc using servo motors. This mill is a little larger then a mini-
mill and about the same size as a small bench top mill like an RF-25
or RF-30.
I'm going to be using a 2:1 belt reduction from the servo to the ball
screw.

First Question: What belt types are people using to do this type of
conversion? XL, MXL, etc...... How wide of a belt? 1/2", 3/8", 1/4
etc???

Second Question: My servo motors will be mounted on plates with
slots so I can tension the belt. I don't know how much these belts
strecth, How much adjustement do my motors need in order to tension
the belt? 1", .5", .100" ?
I'm a little limited in space in how much I can adjust the motors,
base upon my current design.
Thanks for the help!
Bob