There are a few short girls who're pretty successful, but most of them do have influental families. (Adwoa Aboah, Zippora, Cara D from the new Burberry ads...). You need to be 173cm at least, I'd say. Plus perfect body proportions, and an extremely expressive face. Even then you'll have a hard furrow to plough.
Are you from Germany? (as for the open calls....)
2nd Cologne_rocks
There is more pressure today to be even taller, taller ****edited*** than ever before. It's not like in the 90's or early 2000's when say 175/176 was normal/ideal (naomi, linda, cindy, christy, heidi to isabeli fontana etc).
If you are still young about 13, 14, 15, then there is a better chance to get into modeling if you're under 175 and over 173. But once you're 17, 18, 19, agencies expect girls to be at competitive heights, 176-182 with very very little exception anymore.
A girl who is young and short will have to develop into an exceptional model compared to her 178 counterparts. Many times you'll see the shorter girls drop off into the commercial world when they're around 18, 19 because they couldn't compete with 178 girls who can pose, walk and have it all.
Zippora started very young and her mom works at Red Eleven...
Kate Moss was discovered at a very young age and was probably expected to grow a little more but didn't. You'll see that the youngest girls tend to be a tad shorter than the established girls because agencies expect them to grow a little more.