couple things
first recommended is that mx track be no less then 20' wide anywhere.
but of course as wide as you can make it it the best, if you can make the thing 35' ft wide do it!
whoops.
whoops are a pain in the....... to build, maintain, etc. I wouldn't recommend anything but dirt to make them. poles and things have a way to be exposed and then it makes the traction on the whoops variable and difficult.
many make the mistake to only pack the whoops in the direction of the bikes and then the back side. You should pack whoops long ways also and smooth the tops as you do this to make them more uniform. utilize the amount of dirt you use to make each part of the whoops so that it is easy to remake them when you need to.
always make whoops so they drain one way or another, its a pain in the......to groom or maintain whoops with standing water in them.
wardy