Maybe this is more a question for the Rogue Trader forum, but it came up in our DH campaign. So, sorry if it is misplaced:
The party of Akolythes in our campaign captured a small transport ship in one of our last adventures. And "small" means "small", that is 40-50 metres following the description in the adventure (which I loaded down from the FFG page, if I remember it correctly). Regarding to the adventure this ship was capable of interstellar flight. Our Akolythes decided to keep the ship for their personal use at the end of the adventure. Some of them have the necessary skills. In the end I decided to let them keep it.
But later the question started: Is a ship of that size really capable of interstellar flight in the WH40k universe even with a Navigator? The Lexicanum says that the Viper Class Scout Sloop is "among the smalles Imperial starships capable of carrying a warp drive". And the Sloop is definitly bigger than our transport. Most ships in the WH40 universe are hundreds of metres long, often kilometres and have thousands of crew members. On the other hand one can read in some novels like Eisenhorn or Ravenor about small yacht like ships, which are capable of warp flight.
So... can there be starships of that size with a warp drive in "our" WH40k universe? Or would it be better to specify the transport ship as an innersystem spaceship which needs to be transported by bigger interstellar vessels to other star systems?