@Hades_Corps the O-Ring size in all of the Openflexure actuators is I believe the same. The lugs that hold the O-Ring are inside the actuator, so other dimensions of the stage are not necessarily an indication of the distance the O-ring needs to span. The current Deltastage version and instructions were recently updated, @samuelmcdermott would be able to confirm.
The instructions for the microscope V6.1.5, as the current version on the main website, are not as thorough on some of these things as the new version in development - currently V7.0.0-Alpha2. The actuators have not changed, so the new instructions for assembling the actuators may help, particularly the cut-away renders.
@few Viton O-rings are certainly not the ideal tensioner, but it fulfils some important needs. For an open project the band needs to be a part that can be defined precisely and is widely available. O-rings fulfil that, and are actually quite frequently used as holding bands, not just as seals. We have found Viton to be better than nitrile for durability. There is indeed a considerable ‘set’ when the bands are stretched, but that does not matter particularly, as long as the bands still provide tension over the elongation required.
There is another thread on O-rings and microscope resting position