Definately go for the largest tank that you can afford/accomodate as this does make life easier and the tank conditions will tend to stay more stable. The larger the volume of water, the less it can be swayed quickly by such things as a failing heater etc.
For a tank that is 60 litres, just under 14 gallons, you can aim for the general rule of thumb (only applies to small fish) of 1" of fish per gallon. This would take into account the full grown size of the fish, not the size they are from the shop. You can nudge it a bit higher if you up the water changes a bit to compensate. There are a shed load of little fish that most aquarium shops carry as well as neon tetras. Such as glowlight tetras, Black neon tetras, whitecloud mountain minnows, guppys,silver tip tetras, harlequin rasboras, cherry barbs, the list can go on for ever almost. Stay away from things like tiger barbs, buenos aires tetra, flame tetras as they tend to get slightly larger and can be a bit nippy. You might also want to consider something that scavenges around the bottom. Corydoras catfish are good for this, but they are happier in small groups rather than singly. Other candidates for small tanks would be Otocinclus affinis (commonly called an oto catfish). They are great little work horses which should also help keep any algae under control. I wouldn't add them straight away as they are a bit sensitive to new tank set ups, and of course a brand new tank won't have much algae for them to eat.