Good answer above, I would like to add some lazyman's thoughts.
What’s a manageable size to start of with?
Start with small lot even as small as 1 by 2 or 3 metres, and the reason is, without experience you might be overwhelmed by the task.
What about shade... The area I am looking at has a big gum tree to the north so the south side is shaded to varying degrees in summer and winter?
Some shade might be a blessing, but any humangous tree as a veggie patch neighbour spells a disaster to me.
Large trees have established roots system and are on a lookout for nutrients and moisture and can sap up all you put in.
It might even warrant to have raised bed with physical barrier from underlying soil, like pond liner, but this is bit complicated.
What about compost? I've only just started emptying food scraps, etc into a heap at the back, haven't got a bin yet... So far been just emptying and then covering with soil, and so on...
What you do here is not that bad at all, probably horror for purists, but very time consuming what they suggest.
I am so lazy, that allowed I would only use so called sheet composting, which is nice name for laying scraps on top of soil slightly covering it.
Similar method might be collecting food and veggie scraps into bucket and digging it in a patch. All you have to do is remember to dig in next lot in different place.
There are perfect proportions of Nitrogen to Carbon and to moisture, but often close enough is good enough. Generally food scraps will make soil acidic, so handful of lime per 5 litre bucket of scrap should do. As to carbon, shredded paper could take care of that about ¼ of the bucket.
Some kind of soil PH tester at some stage can help to keep soil PH between 5.5 and 7.5
Does the veggie patch need to be raised? Should I put sleepers down to form a raised bed?
Clay soil would be the criteria for raising beds, as it can get too heavy and water logged.
Too sandy soil actually warrants to mix in some clay, so balance again.
How often do you tend to the veggie patch? Is once a week ok, or is this an every day job?
In hot weather even few times daily visit might mean the difference between life and death of plants.
But labour itself actually would comprise of mostly controlling and regulating moisture, plus removing weeds mainly.
Then things like pests control – snails and slugs can be poisoned with pellets, picked up by hand at night, lured to moist traps or even traps with stale beer.
Support of taller plants and fastening them to something as they grow.
Some kind of protection from wind and too strong sun and later from birds.
As somebody said, it can be heart breaking, as there are so many free loaders out there: ants with aphids, earwigs, nematodes, mealybugs, various caterpillars, fruit fly, cut worms, even slaters.