# Set maximum streamline length in VectorPlot for implicitly seeded streamlines

I have written a function that is meant to illustrate the solution of a DE:

ShowDESolution[f_, {xa_, xb_}, {ya_, yb_}] :=
Animate[VectorPlot[{1, f[x, y]}, {x, xa, xb}, {y, ya, yb},

This function, however, has the problem that the length of streamlines can only be set when the streamline seeds are explicitly given. (The max streamline length is the third term in StreamPoints, r.) If I replace the 1 in StreamPoints with {{-xa, 0}}, for example, the animation works just fine. But if I leave the 1 there, the streamline will be the same length regardless of r. Is there any way I could get the optimal streamline seed for that graph and put it in as an explicit seed? Or set the length of implicitly seeded streamlines?