Please don't use sine, cosine, or tangent when solving. Solve it by introducing segments, etc.
The easiest way is to realise that the octagon is made up of eight congruent isosceles triangles, each with two lengths 1/2 a unit long and with their included angle as 45 degrees.
Then the area of each triangle is
Therefore the entire area of the octagon is .
I'm not sure if it can be done without using some trigonometry tbh...
Edit: I misread the question, I thought you wanted the area. The length of each segment is easily found using the Cosine Rule, since you have two sides of each triangle and their included angle.
The length of each side in the octagon is units.