From your question, I understand that you want to plot the variables ‘a’ and ‘b’ against each other.
Firstly, if you want the output variables ‘a’ & ‘b’ to be vectors, then use the dot operator. The ‘./’ operator returns vectors.
Secondly, the variables ‘E_desv_C(:)’ and ‘E_desv_D(:)’ aren’t defined in your question, I am assuming that they are of the same dimensions as of the variables ‘E_D’ and ‘E_C’. If this is the case, the variable ‘err’ will be of size [59x2]. This is why the error “XData must be the same size as YData” is thrown by MATLAB, since the dimensions of the arrays ‘Y’ and ‘err’ are different. To use errorbar(x,y,err), the variables 'y' & 'err' must be of the same dimensions.