To big steppers?

[JAZZCNC]

Can you recommend any products? If I remember correctly the distance between the centers of my ball screws is 684 mm at X.
I was figuring twice that + a little for the pulleys and some kind of tension adjustment device at center.

It will depend on how you setup the tensioning system and idlers. But with 20 teeth pulleys which is good size because gives nice balance of teeth engaged and pulley size which affects inertia of the screw and can have big affect on power requirements. Arranged something like whats in the picture then your belt length will be in the region of 1600mm.