I had the same problem and was able to solve it like this:
[void][System.Reflection.Assembly]::LoadWithPartialName("System.Web.Extensions")
$jsonserial= New-Object -TypeName System.Web.Script.Serialization.JavaScriptSerializer
$jsonserial.MaxJsonLength = 67108864
$Obj = $jsonserial.DeserializeObject($CourseTypesResponse)
You can use $jsonserial.MaxJsonLength
to manipulate the MaxJsonLength property