Every part of the FRC manual is released the day of kickoff, robot rules and all. You can still prepare by making sure people know how to CAD, how programming works, and by building a general purpose driver station, among many other things. You just won’t be knowing about the rules until January.
Last year’s drive station rules will give you a solid idea of where to start. The depth and width of the driver station is unlikely to change every year, and obviously as long as it isn’t excessively tall, you should fit all the criteria.
You can also read the game manual from last year’s game. The game itself changes every year, but the general rules about the robot have stayed mostly the same from year to year (aside from the size limitations). No guarantee about the future, though.
The only rules in FRC released before Kickoff are the administrative rules - awards submission criteria, championship rules, etc. You will definitely not get the robot rules in advance.
You can make reasonable inferences that certain things won’t change, but you can never be certain. It’s extremely unlikely that the field size will significantly change, or the space allowed for a driver station (at worst it will get narrower). So that lets you work on a driver station.
You can use the 2016 build rules to build and test prototypes. You just have to start over (more or less) at kickoff no matter what, and there’s always the outside chance that robot size dimensions, motor allotment, weight, etc. significantly change. But by then preseason work is a sunk cost, so you might as well.