I found this very helpful:
Input field with character counter needed - but one line high only:
(the character counter is only available in field type: textarea - not: text-field)
Tom W. found out the following :
it seems to me that the css attribute min-height overwrites the textarea-attribute rows (the value you enter in the property for the item).
By resetting the css the textareas height works like you would expect, e.g. Thanks Tom W. !
BTW: I use APEX 5.1