Trailer for Fiat 500 behind motorhome

Google is your friend!  Ask 'Trailors for cars' and then sit down whilst you see the prices!

I would expect that 5 meters overall is reasonable although there are bespoke trailers that may be less?

Venture into West Sussex for  a trip to Half Moon Bay Ltd at Storrington 

BJ trailers are good and, if the one you looked at was the right size for the car, then that's the length it has to be. 

If the car needs a deck of a certain length, it's no good buying a trailer that's too short. You need to get the tape measure out.

Universal trailers, Conyhurst, near Billingshurst might be able to help.

you probably already done so, Brian James Trailer site, has a down load able PDFs which describe the sizes and measurements of their trailers. Helpful people, if you phone up and seek advice,

From memory,  the Fiat 500 is one of the heaviest small cars around. (Probably uses proper metal !)  think length of car and add half again for the A frame of the trailer should put you in the correct region of length.

Rgds