Start with people, then luggage
Bigger is not always better. Two to three guests with two or three bags usually fit a sedan; four to six guests or a stroller are more comfortable in an MPV; seven or more need a minibus; thirteen or more need a coach.
Luggage is the part people forget
Four adults with one large suitcase each may not fit a sedan. Tell the operator the number and size of bags, and whether you have a stroller, rather than only the number of passengers.
Travelling with children
Children need child seats or boosters, and the rear seat needs space. An MPV is easier to get in and out of for families with children or older parents; a minibus works for two families travelling together.
If you are unsure
Send the group size, a photo of your luggage, the children's ages and the rough route. A good operator will recommend a vehicle based on actual space, not just upsell the biggest car.
Price and vehicle size
Larger vehicles cost more. Choosing the right size matters more than choosing the cheapest or the biggest.