The issue is that you are trying to do an element-wise division but are not using an element-wise operator. You can see that:
fun(x0,xdata)
returns a row vector, which is not the same size as ydata.
Change the definition of fun and use the element-wise division operator ./ as opposed to /
fun = @(x,xdata) x(1)*(1./(1 + (x(2)*exp(x(3)*xdata))))
