Hi Tomasz Tomczykiewicz,
Greetings from Syncfusion support.
We have checked with your requirement. By Default, our DropdownList component’s item text will be in one line and the long text will be automatically truncated with the ellipsis at the end. We suspect that any of the other CSS style is affecting the dropdown and make to display it as multi-line texts. Please check this at your application level.
We have prepared a sample for your reference,
Please get back to us if you are facing the issue still.