How much space?
The minimum dimensions I reccomend for a pair of gerbils is 100cm x 50cm.
It is very dependent on individual requirements as to the amount of space they need. Factors such as age, quantity of gerbils and how they get along can all contribute to this. Sometimes a very large habitat with different sections may trigger territorial behaviour in some gerbils. Cages around 120cm x 60cm are a great size for providing enrichment, however some gerbils may become territorial. It's about finding that perfect balance for your gerbils.
What Accommodation type?
There are so many options when it comes to housing your gerbils. There are some suitable habitats you can buy online. You can also construct your own or alternatively get one custom built if you don't feel comfortable with DIY. Another option would be to convert a piece of furniture such as a bookshelf into a habitat.
A more expensive cage doesn't always mean that it is more suitable, however cheap pet shop cages often don't provide sufficient space. Finding a secondhand tank (preferably at least 4ft) is a great budget option! Sometimes paying a little extra to get something which will last, and that will give them a better quality of life can be worthwhile. Remember that you should also take into account how well you can see the gerbils in their cage - a glass/perspex cage has many benefits for the owner because it can make owning gerbils much more rewarding pets when you can easily observe them.
There are endless options, but here are some questions you should ask yourself:
Will it meet the minimum size requirements?
Is it safe from other animals (e.g. cats, dogs)?
Is there sufficient ventilation?
Can I clean it with ease?
Is it free from drafts?
Is it made from safe materials which they will not chew out of?
Is it fully secure so they can not escape?