Do canvas trainers run true to size? Mostly not quite, in my experience, they tend to run a touch big in the length but snug across the top of the foot, especially if you've got a higher instep. I buy half a size down from my usual trainer size now, after years of mine flopping at the heel like a clown shoe, and I'd always try them with the socks you'll actually wear them with, because a thick sports sock changes everything on canvas.
The fit problems worth watching for are all at the edges. Check the heel doesn't gape when you walk, canvas has no structure to speak of so if it's loose back there it stays loose, it won't break in like leather does. Watch the sides too, cheaper ones can dig in right at the little toe if the last is narrow, and that's not something that softens with wear, it just rubs. The toe box should have a bit of room to spread, not press into pointed, because canvas creases where your toes bend and that crease is permanent within about a fortnight.
On colour, white is the one everyone wants and the one that shows everything, so unless you enjoy scrubbing canvas with an old toothbrush on a Sunday, go for off white, stone, or a faded navy instead, they hide scuffs and that general grey tide mark round the sole without looking deliberately dirty. Black is the easy option if you want to forget about it entirely. Pale pastels look lovely in the shop and genuinely awful by week three, I learned that one with a mint green pair that turned the colour of a dishcloth after a single rainy commute.
As for ageing, canvas softens and goes slightly baggy with wear, which is part of the charm, but the rubber toe cap yellows faster than you'd think, sunlight does it even sitting on a shelf. A damp cloth and a scrub with a bit of washing up liquid sorts most marks, and I'd keep them out of the washing machine, mine went through one accidentally and came out a strange ghost of their former shape, drum shaped almost, never quite recovered. Stuff them with paper to dry rather than leaving them on a radiator, which warps the sole and makes them rock slightly when you walk, oddly infuriating once you've noticed it.
A good pair earns a particular kind of loyalty, the laces go soft and grey with use and somehow that's the moment they finally feel like yours rather than new.