How can I relably work out the largest cut size of vegatables X and Y so they roast in the same amount of time? I'm interested in making an ultra simple seasonal roast vegetable dish. This site tells me that the vegetables: 


  Beetroot, Brussels Sprouts, Carrots, Leeks, Onions, Parsnips, Potatoes, Pumpkin, Red Cabbage, Swede, Watercress, and Winter Squash.


are totally in right now (but answers should related to any selection of roastable vegetables) 

I'd like to be able to throw some of each in a roasting pan and put in the oven.  However I am aware that each veg will take a different amount of time to cook (assuming I like them all equally soft/crunchy). 

I believe that I can make them all be ready at the same time by cutting them into different sized chunks: leeks should be bigger than potatoes. 

How can I find relably work out the largest cut size of vegatables X and Y so they roast in the same amount of time?  

Ideal answers show me where I can find a look up table that either gives the values, or gives values that I can put through some maths to get the values I'm looking for.
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.