I posted over in the ANOVOS Mandalorian helmet discussion, but it's easier right now to just re-state. Typical human visual acuity is about 40 microns/micrometers. Something with a particle size around there or smaller is better. 325-mesh metal powder is pretty commonly available -- that's about 47µ. Larger particles make for more "sparkle". The other factors are particle shape and carrier. A lot of places don't specify, so you may have to do your homework. In general, though, if two sources of the same size have a significant price difference, guess which is probably preferable. The flatter ("cornflake") particles tend to be better than the rounder ("silver dollar" and "sphere"). Flat makes for a more uniform surface. Round creates multidirectionality that causes glittery light-kicks.
The chemicals used to process those general types also result in different behavior in the carrier, with the cornflake particles aligning in a flat layer at the top, for a uniform metal-looking finish, while the rounder particles tend to be more distributed through the carrier.
I have 325-mesh stainless steel powder I use for surface-coating small things. It looks metal-like enough for a 1:1 small prop of a couple inches. For anything bigger, I base-coat in RustOleum's Professional Grade black (the PG paints have ground up ABS dissolved in suspension as the pigment, so when it's cured, you've basically coated whatever you painted in a smooth, shiny layer of ABS plastic). They I lay down a coat of Testor's Metalizer sealer first, and then hit it with the stainless steel Metalizer and leave it to cure. It buffs to a finish I can't tell from actual stainless steel.
Basically, there are lots of options, but it can take time, effort, and money to find the exact recipe that works best for you and your skills and what you're making with them.