We hired a 400l Kamei roof box Thule bare and for a couple of weeks for our recent trip to France, managed to get two large holdalls, and a couple of boxes of stuff in it. Cost me £100.
If you’re not sure what size you need I’d strongly recommend hiring one, rather that than forking out a few hundred quid and discovering it’s not large enough.
To be honest the cost of hiring one, compared to the cost of buying the same quality of kit, makes me think I’d only ever hire in future when I consider how many times I’d actually use it. If you hire it, decide you’d use it lots then you at least know what size to buy.