You can set the step size tolerance and other performance characteristics to different values by creating an options structure with the optimoptions function. Pass the structure to your fsolve call as the third parameter.
For instance, your code might change to:
opts = optimoptions('fsolve', 'TolFun', 1E-8, 'TolX', 1E-8);
[B,fval] = fsolve(@(b) myfun(b,x,y), b0, opts);
and whatever other options you want to change, although the default value for 'FinDiffRelStep' is likely as low as it should go. Experiment with the others to get the result you want.
