How to tell the size of a bike chain?
Modern chains all have 1/2" pitch (distance between rollers), but there are several different widths. 1/8" is common for single speed and internal gear hubs, while 3/32" (and narrower in the case of are common for derailleur-based bicycles) though many single speed/internal gear hubs use a 3/32" chain (this depends on which cog+chainring combo is used). 

You should check the cog+chainring for a marking of 1/8" or 3/32", and if it has one or the other, pick up a single speed or 8 speed chain respectively. Alternatively, take your old chain and a pair of calipers and measure the inside width between the plates of the chain and see what it comes out to. If theres a quick link in the chain, you can also note that 1/8" quick links have both pins on one plate whereas 3/32" chains have one pin on each plate. 

If you take your chain to your local bike shop, they'll be able to sort you out quite quickly. Note not all chain tools can break 1/8" chains. 

Once you've acquired the appropriate chain, you simply need to find the length of it, which will depend on if you're using a chain tensioning pulley or horizontal dropouts (the chain shouldn't be too loose or too tight, but the dropout gives you some wiggle room for length with this). You can also do this by just comparing number of links with your old chain as well.