The size of the truck you decide to hire depends entirely on how much stuff you have. Smaller houses and apartments will not have nearly as much contents as say a 4 bedroom family home with a second lounge.
Your professional movers will, of course, do their best to help you work out the best sized truck.
There are a few things to think about other than how much furniture you have. For example, if you have outdoor equipment such as outdoor furniture, garden equipment (lawn mowers, line trimmer etc) a portable spa pool and whether or not your children have outdoor toys like swing and slide sets, so don't forget about these things when you are working out what size truck to hire.
Obviously, you do not want to hire a truck that is much larger than needed for the task as this will lead to you spending more money than needed. On the contrary however, you do not want to hire a truck that is too small for your belongings, which will lead to either the truck needing to do two trips or leaving you with the left overs (if the truck has another job booked in soon after moving you) and you needing to make several car trips or hiring a trailer, hence costing you far more money than anticipated.
If you have made the bold decision to Do-It-Yourself, there is a greater chance that you might just hire a truck that is too small and needing to do two trips. This is usually avoided when you hire the services of a reputable and professional moving company. Many furniture removalists have an online quoting system than allows you to enter in each piece of furniture and the quantity of that item, eg 4 beds; 3 sofas; pool table e.t.c.
To find out volume of your house contents (furniture) measure the length, width and height of your combined furnishings (beds, sofas, fridge etc) to find out the cubic meter.
length X width X height = cubic meter (m3)
Don't forget about your boxes however if you have decided to move yourself. The hire company should be able to help with guidance, but they are not professional movers so do be aware of this when calculating how much room you will need.