How to size these resistors for the MOSFET gate or are they redundant for this case?
If the 0/5V_IN is a push/pull output, then you don't need R2. R1 can be a very small value resistor, the datasheet test setup uses 25 ohm. 

If your output is not push pull, then you would need a R2 resistor, that will dicharge the gate capacitance when turned off. Typically a 10k om would do the job.

EDIT:

R1 will determine how fast the MOSFET will start to conduct. If this is not critical, which in your case will just turn on a LED in the SSR, then enough large resitstor can be used. For example a peak current would be I=5V/R1. Using a 500 ohm you get 10mA peak current. This current is present at turn on, when gate cpacitance is discharged. Once the gate is charged no current flows. If you have to turn off, then you might need a discharge rsistor R2. Now you have toy pay attention, since they form a voltage divider you will get a gate voltage smaller thean 5V, so you have to use a ressitor with yet higher resistance. Or you can put the R2 resistor before the R2 in parallel to the output. 

Let's say R1=470 and R2=47k as from your schematics.