With a worksheet object called worksheet, worksheet.Dimension.Start.Row and worksheet.Dimension.End.Row should give you the information you need.
worksheet.Dimension.Address will give you a string containing the worksheet dimensions in the traditional Excel range format (e.g. ‘A1:I5’ for rows 1-5, columns 1-9).
There is a documentation file available. In many cases it might be just as quick to play around with the library and find the answer that way. EPPlus seems to be well designed – everything seems to be logically named, at least.