Corydoras should be able to sift sand with grain size 0.4-0.8 no problem.
I would not go above 0.8 to 1 mm grains, its a bit too big for the corys but I hear its great for plant growth.
0.1-0.2 is very very fine sand and unless you have very specialty fish I wouldnt suggest going for this size. Would also not recommend more than about 2 cm substrate thickness with this sand size.
If youre putting some kind of dirt below your sand, make sure its far enough below so that the corys can not dig it up. A medium sized cory easily digs 2 cm down.
Maybe you could use a courser sand in the back, and have an unplanted area of fine sand in the front. You wouldnt need to put dirt under the sand if you're not planning to plant there, reducing your risk of the corys digging it up. The sand may creep between the course grains in the back with time, requiring you to top up the front every now and then. Or you could do fine grain in the back as well, I think your plants will still get through it ok, especially if you have some tasty nutrients for them to find at the bottom. But you should take my plant growing advice with a pinch of salt, I am still pretty good at growing algae and am a fishkeeper first and foremost.
Or maybe if youre planning to use a fair bit of stones, you can seperate the sand area in front from the planting area in the back. I see the aquascapers using filter floss to plug the gaps between the stones so that the soil doesnt creep through the stones onto the sand.
It is definitely a balancing act when you want to set up a nice habitat for your fish, but also get good plant growth.
---
I got a bit carried away writing this part so I wouldnt blame people if they skip it;
When it comes to corys and sand -in my opinion- there are a few different factors that you should consider when chosing the substrate.
The grains should be small enough for them to exhibit their natural sifting behavior.
I personally find it a bit cruel to deprive them of this option entirely. Its like having a bird that never gets to fly.
A compromise with a designated fine sand area may be a good option.
Another consideration is wether the substrate will be sufficiently oxygenated, so that they do not suffer from barbel loss or damage. I think that barbel loss is more commonly caused by unhealthy conditions in the substrate, rather than course and sharp substrate. This is why you can find people who have corydoras on sharp gravel "without problems" (aside from depriving them of natural behavior), and you will also find people with very fine sand with corydoras suffering barbel loss, because the sand has turned bad.
You also want the grains to be as rounded as possible. I would even say the roundness is more important than the size.
Obviously even if you got tiny razorblades sized 0.1-0.2 the fish would still get hurt sifting it through their gills.
Give the sand a good squint in the shop if you can, or check with others if they know how sharp the grains are in the sand you are wanting to buy.
Its also really helpful to run the sand through your fingers before buying, but this is not always possible.
I know of fine grained sand sized 0.2-0.7 I would not keep corys on, because the sand is made by crushing the stone and thus the grains are sharp even though they are small. I was very unhappy with most of the JBL Sansibar range because of this. It was uncomfortable and abrasive on my skin when I was planting in the tank or moving the sand.
Other sand types will be sourced from water worn sands (not crushed), where the grains are much more rounded and therefore safer for corys, even if you go up a size.
Nerdy part over
---
I may be entirely incorrect, but I thought that "play sand" was a sand made for the sand pits that kids dig and play in.
That it has a larger range of grain sizes, like 0.1 to 1.0 for instance. The wide range of grain sizes enables the sand to pack more tightly without slipping or falling apart, and this makes all the castles and dragons and whatnot stay upright better when little Timmy is playing 😁
It is the same ability that makes this kind of sand unsuited for our tanks. We dont want the sand to compact, so something where most of the grains are the same or similar size will be better for oxygenation, like 0.4-0.8.
But maybe the term play sand is used more broadly or means something else in the UK.
While "Pool filter sand" often has rounded grains and is made in a narrow range of sizes in order for it to be good for filtration. My understanding is that its also usually made of inert quartz so it doesnt affect water values.
I didnt think any of these terms were used for the sands you buy in pet shops, but again I may be mistaken.