“The prohibition is not for you to see it, but for it “to be seen”
on your property, that is, to be there (even if you don’t eat it).”
Right. Just want to clarify 3 points:
1. You are allowed to see it if it is not yours – for example, food in the grocery store- since the problem is not the “seeing”, it’s the owning.
2. If you own it, it’s a problem even if you can’t see it – for example, it’s packed away somewhere or it’s not on your property.
3. When it comes to eating chometz, any amount is a problem. When it comes to owning chametz, if it’s small enough and/or ruined enough, it’s not a problem. Hence, the above discussion about crumbs.
For the exact amounts, you should check with your LOR, as I have heard different things.
I thought that less than a k’zayis was fine, but then I heard that it has to be less than the size of a cheerio.
It may depend on whether or not it is ruined/inedible, or it may be different opinions, or I may have been wrong about the k’zayis.