If I stick to my plans I would be coming from Arundel to Windsor with a change at Clapham Junction. I have never used this station and wonder how large the station might be. I have lots of time to make the connection so it is really just a general question.

There are sixteen platforms (numbered 2-17). Here is a satellite view: http://maps.google.co.uk/maps?ll=51....005932&t=k - CJ has a reputation as the busiest station in Britain. There are TV screens on the platforms indicating which platform the next trains will depart from. But there isn't always a clear indication of how to get to other platforms from where you are. The Brits could learn a lot from Deutsche Bahn in this regard.

You didn't ask this, but it might interest you to know that National Rail suggests that wheelchair users allow an hour to change trains at CJ.

It is a large station, as you can see from http://en.wikipedia.org/wiki/Clapham_Junction. It claims to be the busiest railway station in UK, and now in Europe.
It has 16 platforms, but there are electronic destination boards on the overhead gangway, indicating which platform you have to go to catch your train, so it's not too bad. There are also small cafes and shops situated there.

It is a big station, however trains pretty much stick to regular platforms for most routes.

While big the layout of the station is very simple

If you can't make sense of the information on the screens (which is again simple)ask a member of staff which platform for the next train to Windsor, and they will point you in the right direction.

At Clapham Junction, the trains to Windsor leave from platforms 5 or 6, which are actually opposite sides of the same platform. For connecting between platforms, there is a footbridge at the western end and an underpass at the eastern end. Kiosks selling coffee, etc, are dotted around the station. There are two or three trains to Windsor every hour, so you won't have to wait long.

On reflection, Arundel to Windsor by train via Clapham Junction? Via Hove/Brighton?

Sounds long to me. The Arundel-Brighton line is super slow with those delightful trains you see in the black and white movies, drive!

Trains from Arundel to London do not go via Hove or Brighton. You'd need to change trains to get from Arundel to Brighton, because Arundel is not on the coastal line, but on the scenic line which goes up the Arun Valley.
There are two trains every hour from Arundel to London via Horsham, Gatwick and Clapham Junction.

Thanks for the information. Nice to know about the platform numbers.

I changed trains at Clapham Junction to go to Windsor. It is a quite large station, but simply laid out. I confirmed with a station attendant on the platform to make sure I was waiting at the correct platform (to double check what I had read on the digital display.) He called me when the train arrived to make sure I did not miss the train -- very kind of him.