'1. The size requirements are based on distance the same way warps are, so if the warp needs to be 10 conduit blocks for the world distance, a portal between them also needs to be 10 conduits.
2/3. The portal that you press "open portal" on is from that point on, the "source" portal, and to press "open portal" you have to have a token to another portal, which once the portal is opened becomes the "destination". The destination is passive in the process, it simply receives a connection from the "source" and can be totally empty with no token or shards and takes no part in the fueling of activation costs.
To steal a reference, the portals are just like Stargates, where one stargate could connect to another even if the other had no power whatsoever, and the starting point of the connection was required to keep powering the wormhole, only that instead of 'dialing an address', the 'address' of the destination is stored in the token you take from that destination