For a towable, I suspect anything over 2 GPM (at your rated engine speed) will be fine.
Running higher pressures is the simplest way to get better performance, so if everything else in the system is rated for at least 2000 psi that's a good place to start. So that has to be all the hoses, cylinders and fittings, not just the pump and valve...lowest rating sets your safety limit.
Do you have an engine yet? If you are starting with a certain horsepower, go with the pump size suggested by
https://www.surpluscenter.com/tech-help/tech-help-home/ They are pretty simple and straightforward and cover most of the basics pretty well, also their prices are reasonable, especially on hoses.
Most valves will start at about 10 GPM in size, which will probably work pretty well for you. As a general rule, you want the valve rating no smaller than the pump output, but something that is still close is better than a huge valve with a tiny pump, but they only go so small so at some point you just have to go with what you can get.
And lastly, if you find something moves too fast to be easily controllable after you build your machine, remember that you can always slow something down in either one direction or both with a needle valve. Something that moves how you expect it when you pull the handle is better than always trying to feather the valves and forgetting half the time and sending the bucket slamming into things.
Have fun, and remember, if something really isn't working, you can always change it, that's the joy of building your own machines.
