Depends on several factors. The first thing is to decide what gear range you want.
Then you can usually get there by changing either the sprocket or the chainring, or maybe both.
If you want to fit a ChainGlider, that limits the options. There are only a few possibilities for compatible chainring size, Chainglider front parts are 38, 42, 44 and 48 IIRC. Same goes for sprockets, with the constraint that smallish sprockets need the special Rohloff compatible Chainglider rear part, which IIRC is different for threaded and splined sprockets. Large sprockets 19,21,22 are in practice compatible with the standard Chainglider rear part as used for Shimano Nexus 8 hubs. Another thing to consider is that Chainglider front parts aren't necessarily compatible with all the rear part options, IIRC the 48T front part won't work properly with the 19-22T rear part. Chaingliders also work best with thin chainrings, the Surly stainless steel chainrings work well for me and are reversible. But I have one bike fitted with a wide TA ring using 1/18" chain and a 1/8" sprocket, this works OK with a Chainglider but needed a "breaking in" period.
Changing the chainring can be economique if you have a stock of spare chainrings. I did this with my Raven Sport Tour, swapping out the 50T TA ring for a 42T TA ring I had in my spares box and leaving the 19T sprocket in place. This also allowed me to fit a Chainglider as I then had a 42x19 combination.
When I lowered the gearing on my Raven Tour I kept the 38T Surly chainring and changed the sprocket from 16T to 21T. I also had to change the Chainglider rear part.
One minor consideration is that for a given gear range a large sprocket/large chainring combination wears more slowly than a small sprocket/small chainring combination. That was the reason I originally chose 50x19 for my Raven Sport Tour. But a transmission that is partially protected by a Chainglider should last a lot longer than an unprotected transmission.
