It's all going to depend on how you intend to use the bus. You need to ask yourself these questions first. Do you plan to travel all the time? If so then you will need to have ample water storage for drinking, showering, and possibly using the toilet. You will also need to have ample battery storage for your electrical needs, whether it be 12v DC or using an inverter for 110v AC. The more you plan to travel the bigger the bus you will need to equip all the necessary items.
If you plan to stay in RV parks the majority of the time then you will have the use of the water, electricity, and shower facilities when parked and can get by with maybe a smaller bus.
Last but not least, will you be using the bus for just a weekend or maybe a week long getaway, or do you plan to live in it full time? Will there be children or pets with you? If living in it full time then you will really appreciate the biggest you can afford because even empty the biggest ones are still only around 230 sq. ft. of inside livable space.
Read up as much as possible in the Skoolie Conversion Forum here. It may help to answer your questions. Good Luck !