That FAQ link that Therm posted is a good link to check through for future queries, another good link is the list of stickies:
I think you mentioned being dyslexic in an earlier post, so I'll summarise for you.
That cage looks to be a good size for two birds.
The recommended minimum cage size for 2 birds is 30 x 18 x 18 inches.
In centimetres that is 76 x 46 x 46 cm, which gives a minimum cubic volume of 161,00 cm3
For an occasional 5 day period, I agree that you could get away with a smaller cage, providing your two hens continue to get along well.
I think it will probably be best, if you find a good spot for the cage in the caravan first of all, and then look for the biggest cage that you can comfortably fit in that spot.
*You might like to look at the vision cages for the caravan.
They tend to be a love it or hate it design, but they ARE really good at keeping all the mess, seeds, feathers inside the cage, which would be extra handy in the confines of a caravan. I've often seen them for sale second hand on gumtree too.