A rule of thumb is that a reamer can remove about 0.5% to about 1.5% of its diameter; for your 6.5mm hole this means the drilled hole will be between 6.48mm and 6.40mm. Stay closer to the larger drill size for small holes because small reamers are not as strong.
It is possible to force a reamer to remove much more than this but not good practice. Also if you are working with an adjustable blade reamer take even less per pass.
With blind holes an adjustable reamer cannot be used but even a regular reamer will not ream a blind hole to size the entire depth. The leading end of a reamer has a slight taper and does not cut full size until about 60% of the diameter back from the end.
A final note with reamers is never, never ever rotate them backwards. If they are intended to cut in a clockwise direction turn them clockwise both going in and out. And don't turn fast and go in slow; run a reamer at maybe 10% of the speed you would drill a hole, use plenty of cutting fluid and feed at up to 10% of the diameter per revolution.
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