How to get size of SDE geodatabase feature class

No, there is no standard way to determine this information, even with respect to a single RDBMS implementation. Even when you poll the database for the storage of a table, that might not correspond to the actual disk use (due to blocking factors, high water mark, etc).

I'm a dba rather than a GIS guy, so I hope this is useful.

I respect the comments from the other users that getting this info from a db can be difficult or misleading, however assuming you have sql query access you should be able to work this out for most if not all db implementations.

What database are you using? I assume a feature would translate to a single row in a table. Are you looking for an average? The total for the table, or kb for a specific row?

Assuming sql server: sp_spaceused tablename will give you kb's used, rows, used space and unused. So you can work out total and average.

If you can identify a specific row you could select that into a #temp table and run sp_spaceused on that.

A table will use a minimum of 1 page or 8kb. So if your feature is less than that it might be harder to calculate. I guess you could insert the same row n times and then divide by n.

Can you tell us more about what you are trying to achieve and/or why?