It’s rare these days, unless working on bicycles from mid-20th century and older, to encounter any bolt/nut that isn’t metric. On the contrary, it’s somewhat common to see the head of a metric hex bolt that has been rounded due to a standard sized hex wrench being improperly applied.
Most hex sizes found on your bike will be in the 3mm-8mm range. On occasion smaller sizes are used(shifter covers), and sometimes larger(freehub body).
It is crucial to use the proper metric hex wrench on your bicycle as to prevent your bolt heads from rounding out.
There are two exceptions, at least within the 3mm-8mm range we discussed, that won’t damage your bolts in case you only have a standard hex set.
4mm is practically the same exact size as 5/32″.
8mm is practically the same exact size as 5/16″.
Please take caution when using standard hex wrenches on metric bolts. If you feel as if the hex wrench you are using is slightly too small, stop wrenching immediately and determine which wrench is appropriate for your specific application.