The portals we talk about here are the ones that are now connected to the 'portal network' that sprang up around the time the Grand Mother was defeated. The Peculiar Portalist knows this system best, but she is not a very reliable source of information unfortunately.
This is all the information we currently have:
We don't really know. It has to do with magic and intertwining strands of the weave of the arcane plane which are then connected through the realms to span the distance in our material plane.
No, apparently it is a closed system of portals that have been connected in the past and are still intertwined.
Yes, though we found that not all portals are connected to all others... why: we do not know.
The use of portals is heavily restricted by the Eldritch Inquisition who monetizes the traveling from one location to another. The Eldritch Inquisition has dominance (by physical force) over almost all known portals in the big cities though this is disputed by the other guilds (mainly the Commerce Factions because they need to pay to transport their goods)
An arcanist with enough knowledge of portals can make a mental connection and weave the flow of magic in such a way that they can sense the destinations and grasp the thread of it to connect portals to each other. Most portals that have been discovered are now manned by skilled 'portalists' as they are colloquially known to provide these services for a fee.
The physical dimensions of most portals are about 3 meters (about 10 feet) width and height. If there is a limit on the mass we have not yet encountered it but loads of ore as heave as several tonnes have been moved through so we speculate there is none.
Portals that occur naturally, like a mushroom circle through which you can go to the fey, do not interact with the portal network we are describing here. The same goes for teleportation circles or portals made by spells. They do seem to work on the same principles of magic and interact with the weave as well, but they are from a single point to another specific point while the portal network makes it possible to go from any portal to any other
activated and known and often used portals
hidden and activated; maybe someone knows about them but not often used
known portal but not activated
not activated unknown portal