It should be same internal size as new plastic soil pipes. You could use a standard 4" internal cap. It will have a ribbed rubber outer seal. Best get as heavy duty one as possible, but any will do.
Probably about 4-5m from the manhole. I'm assuming that other things in the house use the same feed into the manhole - but I would need to check to be sure. There is a WC close to the soil pipe in the photo, so I assume these connect together somewhere.
Be aware, with old clay pipes joints are solid, sand/cement, and pipes are relatively fragile. Not unusual when breaking into pipe, for adjacent pipes to fracture. Unless there is space and inclination to excavate and relay broken pipes keep disturbance to minimum.