There are some commonly occurring lengths for multi-band dipoles. I see
105', 135', and 140'. Is 140' preferred over 135'? If so, what is the
benefit of going from 135' to 140'? Just curious.

Pse look at the web: http://www.qsl.net/aa1ll/cfwire.htm
Depending on your location (length of the feeder and free space for the
dipole) consider Region 1, 3 or G5RV Region.

73 Chris SP7ICE