its should say in the manual of the sub in litres or cuft (cubic feet). One Called Sealed box (an air tight box once the sub is fitted)and the other ported (one that allows air to flow) and should suggest the measurements of the box for each for example ...
400 mm,440 mm,300 mm,140 mm
Usually for lower range subs like yours it will be 1.0cuft/28.3 litres for a sealed usually. You can buy them online quite cheap dont bother will any that cost more than £40 unless they are custom made for ur car/sub mdf is cheap shit and website talk alot of crap claiming their cheap mdf aint cheap mdf but good mdf theres no such thing theres shit mdf and shitter mdf but as long as its sealed properly it will give good sounds.
Hope that helps ignore any typos lo.