I am making a sculpture with a pure copper wire wrapped with polymer clay. Will I have problems baking this in the oven with a temperature of 120-13 degrees Celsius/ 248-266 degrees Fahrenheit?
Copper has a melting point of 1085°C/1984°F which is well above your expected temperatures. There should be no danger of losing structural integrity of the sculpture due to melting of the copper wire.
According to the chart at hyperphysics, copper has an expansion coefficient of 17x10^-6 per degree C. This is 0.000017 or 0.0017 percent per degree C. Only aluminum, brass and silver expand more than copper. If you consider an extreme case in which your copper was formed when the temperature was 200° cooler, this would make the expansion 0.34 percent (if my hasty math is correct). 33 percent is one third. 0.34 percent is one-three-hundredth and that's for a two hundred degree change.
There will be expansion, but I expect that it will cause no impact on the sculpture.