The default CSS themes (white, green, transparent, etc.) and themes created using http://themes.daypilot.org should not affect the event dimensions.
When adjusting the theme manually please remember that the event is created using two divs (_event and _event_inner). The dimensions of the outer div (_event) are set by DayPilot. For the inner div, you should use absolute position with dimensions related to the outer div (i.e. top, left, bottom, right) and overflow:hidden.
Example:
.calendar_white_event_inner
{
position: absolute;
overflow: hidden;
top: 0px;
bottom: 2px;
left: 2px;
right: 0px;
}
Let me know if there is still any problem.