As claudiusx said, normally pinkies aren't a good food for beardies.
Pinkies are much higher in fat content than a beardies normal diet.
Some breeders do sometimes give the occasional pinkie to a
just-laid-her-eggs breeding female to help her put weight back on quickly,
but even then it's just every once in a while and not done often (too much
fat can cause all kinds of liver issues).
If, however, you're determined to do it, then you need to at least wait
until the pinkie is smaller than the space between your dragon's eyes.