Super open ended. depends on travel numbers, Whether you have a motion ratio, etc. Look at an ultra4 car to get an idea. If on the axle, same length works. If on a trailing arm where there is some ratio, BP typicdally goes behind the coil carrier, but a 16" coil carrier will pair with an 18" bypass and, on an arm with ratio you're likely 20+, that's a lot of travel if you aren't soaking up whoops.
So, again, super open ended. If mounting on the axle and 16" is desired, if you can fit it with steering and stuff, same length.
Another thing to consider is, "Do I need bypasses." Decent chance of no, if you buy good 2.5" co's and pay a tuner to show up and spend a day throwing magic at it you'll get more out of it for less cost than buying BP's, and not including mounting them.