If you mean the default tooltip from a datagrid.
Yes this is possible with CSS.
You can add the following element to your style sheet.
.mendixTooltip {
width: 800px !important; /* Whatever size you like */
}
Thanks for the answer; this works! (I had a (i think caching issue) but now it works).
One additional questions on this topic - is it also possible to configure the time the tooltip remais visible? Now, when I mouse-over, the tooltip disappears imediately. Can I set this to 1 or 2 seconds?