The decision between 700 cm and 26 inch will have to be made early in the frame shopping process. Brake mounts and frame geometry can not be changed.
There are essentially two choices in wheel sizes when it comes to standard (not recumbent or other) touring bicycles. The width can vary but the diameter is either 700c or 26 inch. I do not know how one got to be measured in metric and the other in inches. To my knowledge this is only done in North America. Typically mountain bikes use 26 inch and road bikes use 700c wheels. 26 inch wheels are slightly smaller than the 700C wheels.

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