I think before you decide on how large a bus you will need you need to first decide what is the most important stuff you need in your tiny home on wheels.
If you really want that king size bed, soaker tub, a gourmet kitchen, and a comfortable living room you are going to need the longest bus possible and add some slide out rooms to accommodate all of your wants and needs in regards to space.
If on the other hand you want to stealth camp on the streets of a city the smaller the outside the easier it will be to hide in plain sight.
Somewhere in between is what you most probably will want.
In regards to how hard it is to convert, I don't think it is fundamentally more difficult to convert a 40' bus than a 24' bus. The difference is just a matter of scale--square feet of flooring, cubic feet of insulation, square feet of wall and ceiling covering, etc.
Regardless of size there are some design requirements that won't change regardless of size--electrical, plumbing, HVAC, appliances, basic cabinetry, etc. You may have to have more or larger parts and pieces but every conversion has some form of basic house systems.
Whether you go minimalist to start with or go all Country Coach in your conversion, at the end of the day it is going to be your tiny home so make it the way you want it to be.
Tango and Ol' Trunt have done amazing things with old buses that are not very large.
aaronsb, Brown Crown, Elliot Naess, Eastcoast CB, soujourner, MuddaEarth, yeggs, gmarvel, and many others have detailed some very nicely done conversions on full size buses. Each one has brought to their conversion a unique spin that has each of them with significantly different end products.
I wish you the best of luck.
Good luck and happy trails to you!