The maximum number of characters or words for the abstract text field can be set for each submission type / conference track separately.
Please go to:
Overview => Settings => Manage Submission Types / Conference Tracks
Select a submission type and click on "edit".
Scroll down to the section "Settings for the Submissions Form of this Type or Track"
Select the appropriate number of words or characters for the option "Maximum Length for Abstract".
Please note that you can select either words or characters when scrolling up or down the list (see image).