What size caravan should I buy?
You need to consider the MAM and GTW of your vehicle, these define the weight you can actually tow and stand for Maximum Authorised Mass and Gross Train Weight, they should be written on your vehicles VIN plate or freely available to find on google, once you know how much you can tow, you know what weight of caravan you can buy, most good caravan sales will note this on the listing, if your buying privately you can see it on the chassis, ask the owner to point this out to you, caravanning people are really helpful!
The second consideration for what size caravan, is how many beds you will need, a family of 5, two adults and 3 kids, would require a 5 berth caravan, typical examples for a family setup are bunk beds at the back, a side table that converts to a single bed, and the front end that is either two singles or converts to a double bed.
However having a smaller car and a big family is not uncommon, if you need a lighter caravan but still want to take a big family camping you can consider an awning, this is great for the warmer months, unfortunately it can get pretty cold if your going out all year round so that's something to consider.
How can I find the MAM and GTW of my vehicle?
How much weight can I tow?
Does my drivers licence let me tow a caravan?