By default, events are not stored anywhere so you need to reload them (use DataSource and DataBind just like in Page_Load):
Public Sub dpCalendar_EventResize(sender As Object, e As DayPilot.Web.Ui.Events.EventResizeEventArgs) Handles dpCalendar.EventResize
Me.dpCalendar.Resources.FindById(e.Resource).Columns(0).Html = 20
Me.dpCalendar.DataSource = ' ....
You can also store the events in the ViewState but it's not recommended because it's much slower (StoreEventsInViewState property). In that case the data binding wouldn't be necessary.