With the Spark API you can only set the content length for the response (package spark, class Response, method header()).
In Jetty you can set the attribute org.eclipse.jetty.server.Request.maxFormContentSize (Jetty documentation) and change the limit of the form.
Currently it's not possible to change directly the Jetty settings through Spark, however there is an open issue in github (issue) to solve this problem.
Update
Now it's possible to configure the Jetty server (source).