[dcwired]

The fixture wire has 1 conductor with smooth insulation which is the hot and 1 with ribbed insulation which is the neutral. If you are not sure which is which, don't fret about it, it will still work if you get them reversed.

 

[dcwired]

FYI, the reason there is a designated hot and neutral on a light bulb socket is so, if the switch is on, one is less likely to get shocked if the shell were hot (normally the neutral connects to it) and you touched it changing the bulb. No effect on its operation.
Larger orange wirenuts are ok. I would use yellow.