user 970953, vlethakula has provided you with a query that will give you the table's allocated size.
I will add a couple of other considerations since there are many reasons to want to know how much space a table is using such as if you need to duplciate it on another system. The table may have associated indexes which in turn have storage allocations. The table may have LOB columns and each log column will also have a storage segment and depending on why you are looking at the table size you may also have to include the LOB segments in your calculations.
The query also does not identify how much space within the allocation is actually being used. You can estimate that by looking at information provided in the dba_tables view specifically using the blocks, avg_row_len, pct_free, avg_space, and num_rows columns.
You can find information on the rdbms dictionary views in the Oracle version# Reference manual.
A quick summary of the basic view can be found in the following article.
How do I find information about a database object: table, index, constraint, view, etc… in Oracle ? http://www.jlcomp.demon.co.uk/faq/object_info.html
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