How can I relably work out the largest cut size of vegatables X and Y so they roast in the same amount of time?
First, there are two approaches. I like to cut things into the size I want to serve (for roast potatoes each piece about 2 or 3 bites), and cook them differing amount of time. This lets me prep veggie #1, get it into the oven, and start prepping veggie #2. Or you could take your strategy of prepping them all at slightly different sizes and putting them all in at once.

Either way, you'll start with a decent guess [*], and monitor as they cook. Some things will be ready too soon - you can just pull them out and have them wait for the others. Some won't be ready when everything else is and will get some time on their own at the end. Either way, you need to update either on paper or in your head that veggie X needs to be cut smaller (or put in sooner) or cut larger (or put in later.) 

* hardest need to be smallest or first: I would try Potatoes, Winter Squash, Pumpkin, Swede, Beetroot, Parsnips, Brussels Sprouts, Carrots, Leeks, Onions, Red Cabbage. Watercress I would barely cook and you can't change the size. Toss in for the last minute or so.

Next time, use your adjusted sizes or times. Repeat until it's perfect every time.