Can you help me estimate in suitable metric units the length of a fly

Answer 1

How many flies lined up make a meter ?
I think maybe a couple of hundred
so
1/200 = 0.005 meter (I would call that 5*10^-3 meters)
but you could also call it 5 millimeters or half a centimeter or whatever if you are into non science units

Answer 2

I'd measure it in millimeters.

I hope you have a ruler that has millimeters and centimeters. How about a meter stick with those markings?