If you use the 'Search' button (above) and type in 'Jockey wheel' you will find lots of threads where this topic has been discussed.

The general concensus is the wider the better to reduce the risk of the wheel digging in and it being easier to push/pull. The pole diameter should be 48mm.
For about 8 or 9 years I used a pneumatic wheel and this made life a lot easier., although some users have experienced puncture problems though.
You can get a wide solid "tyre" or if lucky you may be able to track down an Alko Premium jockey wheel with built in noseweight gauge for a sensible price. I went this route on my current van as the extra cost on a new van was not excessive.

Colin

Hi Steve.
We've had an ALKO soft jockey wheel assembly on our Troll for 10 years and it serves us well. I can't help with the measurements unfortunately but the size you quote does seem to be standard.
Randa