If all else fails pick up a plastic digital caliper they are fairly inexpensive and measure the thread it will be metric and slightly under the size.
EG. a m4 bolt will measure around 3.9mm at the threads.
The small metric bolts generally have a standard thread so you only need the diameter they do not have a fine or coarse thread like fractional fasteners do.
If you are ordering bolts order by the diameter that would be designated by a M and then the diameter in mm. For the length measure the thread length not the full length of the bolt.
Hope that helps you some.
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