Resize a biped with skin already weighted

Make sure all the envelopes and vertices are weighted at the original size.

Make sure you have a simple test animation.

In skin, turn off Always Deform under Advanced Parameters rollout.

In structure mode, scale up and position the entire biped with the scale tool.

Exit structure mode -- Go to frame 0 in animation mode [not structure mode]

Exit biped and scale up and align the mesh with skin to match the new scale and position of the biped

Then, turn Always Deform back on --- now the mesh will deform to the test animation.

If the mesh does not deform correctly, it is probably not in the perfect position -- turn off Always Deform again, move the mesh a little, turn it back on, and scrub the test animation again.