Right now it is mostly just a point on the sky, it is difficult to tell if it is active (like a comet) yet. If it is not active, IE: asteroid like, then the current observations put it somewhere between 8-22km in diameter (this depends on the albedo of the surface). From what we know, we would expect it to likely be made up of darker material meaning given that range of diameters it is more likely to be on the larger end. However if it is active, then the dust coming off can make it appear much larger than it is. As it comes in closer to the sun and starts to warm up it may become active (or more active if its already doing stuff).
It will not pass particularly close to any planet. It will be closest to the sun just before Halloween this year at 1.35 au, moving at 68 km/s (earth orbits at 29-30 km/s). It is also retrograde (IE, it is moving in the opposite direction of planetary motion), for an interstellar object this is basically random chance that this is the case.
Link to an orbit viewer: https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=3I&vi...
The next couple of weeks will be interesting for a bunch of people I know.
Source: Working on my PhD in orbital dynamics and formerly wrote the asteroid simulation code used on several NASA missions: https://github.com/dahlend/kete