I seem to do nothing but opus floors these days. I do a lot of work for national Stone retailer and most of the time the small squares are always bigger than the rest. The company always cover themselves with loads of paperwork, on delivery. In the paper work, for the customer and fixer, it says the tolerence in the joints,for opus, is between 3-10mm. I aways lay out the 14
tile patteren dry, check the varation in the joints, take the measurements and fix it in pannels. Yes all the joints are not the same, but this is part of the effect. This does not apply to anything that is polished, these need to have pefect joints. But for most stone with distressed edges it is fine to have some variation in the joint sizes. You never cut stone down to fit it in. This is a sign of bad setting out. If the sizes are way out, yes then send them back.
Fixers and suppliers should know this and tell the customer before they start fixing what the finished area will be like. Then if the cusomer is not happy with variation then they can take it up with the suppliers. But to cut tiles down takes a lot of extra time (do you charge extra for this?) and does not look very proffessional.This is only my opinion. But I do these kind of floors everyday, not once a month, on £5-£10mill+ properties and I have never had any complaints.