The drive circuit as shown, if 'logic' might be TTL at 3.5V, is
inadequate. The LED in the opto takes circa 1.2V, the visible light
LED takes maybe 1.6, maybe 3 (depends on color), and the base-emitter
drop takes 0.7V; that doesn't add up and give you a margin of operation.
Try using the high gain of the transistor to drive parallel loads instead of stacking everything in series, like:
simulate this circuit – Schematic created using CircuitLab
As to the 'R3' value, with the mininum CTR of 4N35, 40%,
and feeding circa 10 mA to the optoisolator, you get minimum
of 4 mA base current. The 5V, 70ohm
relay will take 71 mA, so a switch transistor with beta>100 and
200 mA capability (2N3904 or lots of others) will work fine.
Figure on wasting one or two volts, with R3 * 4 mA = 1.5V
and you get R3 = 360 ohms. At 5V that (if CTR is very high) limits
the isolator transistor to 12mA, which won't hurt it or the base of Q2.
4N35 collector current maximum is 50mA continuous, 100mA peak.