in the webassist/validation_tooltips/validation_tooltips.css file, edit the width of the .WA_SpryValidationTooltip_errorWrapper selector:
.WA_SpryValidationTooltip_errorWrapper {
border-style: solid;
padding: 10px;
white-space: nowrap;
width: 100px;
}
so it will be wide enough to fit the error message.