Even a Sidey can fit a 2 passenger economy class cabin so that would be entry level, if you want economy, business and first class cabins you'll need something bigger like a Cobra 3 or a Corvette, for luxury class cabins you need one of the dedicated passenger ships the Dolphin, Orca or Beluga.
Retrofitting a ship with cabins means you need to use EDDB stations to find a station that stocks the cabins you want, I generally run a mix passenger and cargo missions on whatever ship I'm using if possible
https://eddb.io/station
Seriously?EDDB collects station information from participating Commanders and uploads the data to the EDDB website then we can find stations, trade routes, commodities, ships, equipment etc, the station section means you can input your current location, the parts/ships you want and it'll generate a list of nearby stations that stock them.
The equipment "codes" can be found on Coriolis https://coriolis.edcd.io/ so an economy class cabin would be 2E to 6E for slot sizes 2 to 6 just input the code into EDDB stations and your sorted, it works the same for weapons, FSD, thrusters etc
You'll need to get familiar with several ED utilities like EDDB, Coriolis etc as they make playing the game a lot easier, I've put together a few images for EDDB stations so you can find the modules yourself, after all if you can play ED then using EDDB is easy!
Thx guys!
I've been playing since pre-release Beta (about 3 years) but I still remember how in' steep the learning curve is in fact I nearly gave up on the control system at one point, I reckon the trick is to take it slow and get familiar with all aspects of the game rather than trying to rush one specific type of game play (also known as "the grind")