A quoted diameter without a tolerance tells you too little about how the ring will fit. The same applies to the cord cross section and its joint.
Ask for the tolerance on the finished internal diameter and the cross section. Ask whether it applies around the whole ring, including the joined area. The manufacturing method matters. Extruded cord and moulded rings do not necessarily have the same dimensional variation. A very small change in cross section can change squeeze in a shallow groove.
Then compare the proposed ring with the actual gland at its tightest and loosest limits. The useful question is whether the assembly will still have enough squeeze without overfilling the groove. The answer depends on the material, pressure, temperature and whether the ring is stretched during installation. A drawing tolerance copied from a smaller moulded seal may not be achievable for a large joined ring.
For a critical installation, agree how the finished part will be measured and accepted. Ask the supplier to state the tolerance class and inspection grade on the quote. The CO.SM.O step moulded route is normally supplied to ISO 3601-1 Class B with ISO 3601-3 Grade N inspection. Class A and Grade S, and optical inspection, are available on request. Do not use an unspecified claim of precision as a substitute for a number on the drawing or quotation.
Related: large diameter O rings, what to send for a large O ring quote and the guides library.