I know the the double radiator could be smaller due to its greater output but I wiould just keep it the same size and fit the new double one with a thermostatic valve to control any excess heat the new one would have.
If you did want to work out what size you would need information such as size of room it has to heat, wether it has any external walls,windows etc. there are variuos charts which can help you work it out - type in " btu " into google and follow the links.
Agree. In addition, mucking around with the distance between the feed / return pipes isn't worth it. If you fit one the same length, you won't have this as a problem.