The chainring is the vital component that transmits your energy from the cranks to the rear wheel.
The size of the chainring is generally proportional to the number of teeth. Therefore, it is common to describe a chainring in terms of the number of teeth. Eg. 56T. As the teeth increase, it means higher gear thus harder to pedal. Though it is uncommon to see foldable bicycles with dual chainrings, it is still common in road and mountain bikes. For example, some road bike riders still favour the 53T-39T.
One of the reasons why foldable bicycles shun away from multiple chainrings is the weight. Foldable bicycles are designed to be portable therefore weight is an important factor. A single chainring setup means no front derailleur, single shifter, and fewer cables to obstruct folding.
Does it mean it is recommended to get the largest possible chainring?
No. It will have to depend on the application and usage. A big chainring means it is harder to pedal. On the other hand, a small chainring will mean easier to pedal. Therefore it is common to see mountain bikes with small chainring as these bikes are required to overcome uphills.
Having said that, chainring forms part of the equation affecting gear ratio. If given the same freewheel, tyre size, and cadence, a larger chainring is theoretically able to achieve higher speed. This is because, for every turn of the crank, the rear wheel turns more revolutions.
For example, a 56T chainring couple with 11T freewheel, 37-451 tyres at 90 rev per min will have a theoretical speed of 45 km/h. It would have been 42 km/h if using a 52T chainring.
Are chainrings interchangeable?
Broadly speaking, yes. As long as the new chainring is compatible with the existing chainset. Nevertheless, every foldable bicycle is designed differently. You may be much better off selecting a suitable bike initially than upgrading later. This is because the additional labour fee does not come cheap.