In general, an empty CLOB uses 1 extent, which is 64K on most new databases.
To understand why this is, recall the fact that each CLOB is an additional segment of an existing table. For example, a table with 2 CLOB columns has 3 segments: one for all non-CLOB columns, and two for your CLOBs.
Each new segment uses the storage properties of the tablespace, unless you specify segment storage parameters. In my case, my application tablespaces are locally managed with autoallocate, so all of my CLOB segments start with 1 extent that is 64 KB. You might have completely different results.
Query the view USER_SEGMENTS to find the names of your CLOB segments. For example, SELECT SEGMENT_NAME FROM USER_SEGMENTS WHERE SEGMENT_TYPE = 'LOBSEGMENT';
The names can be ugly system-generated names like SYS_LOB0000045854C00004$$. Then, run the query like this example:
SELECT BYTES, EXTENTS, INITIAL_EXTENT, NEXT_EXTENT
WHERE SEGMENT_NAME = 'SYS_LOB0000045854C00004$$';