Any question that can be paraphrased as "How do I do X in [Progress | OpenEdge]?" should include a statement about the specific version you are using, down to the service pack/update, i.e. the content of the file $DLC/version (%DLC%\version on Windows).
It is a seemingly simple question, but there is more complexity to it than you might think. Before we can meaningfully answer your question, we need to define what it means. In other words, we need to ask a more precise question.
What does table size mean to you? It could be interpreted in different ways. E.g.:
Logical record size, e.g. record count * mean record size
The size on disk of the table's cluster chain, if it is in a Type II area
The size on disk of all of the record blocks that contain at least one record from the table, if it is in a Type I area
Any of the above, along with the size (logical or physical) of all of the table's indexes
Some combination of the above plus the size of the table's LOB columns, if any
That would beg the question, what is the size of a LOB?
In some cases, for some versions, there may be utilities that provide these answers. In others, you would have to run some custom code to get the answer. And the answers can be affected by the current configuration of your database. For example, imagine you have a table with a mean record size of 100 bytes and it contains 1,000,000 records. You dump the table's schema to a .df and the data to a .d. Now load those files into two different databases: in one, the target area has RPB 128; in the other, it has RPB 1. In both cases, the data is logically identical in size. But in the latter case, the table will consume dramatically more disk space.
For starters, it might help to know the question behind the question: why do you want to know the size of a table? What do you plan to do with this information? Knowing that might help us to help you.
You might also want to know the size of the indexes associated with that table. It is unlikely that they are of zero size and if size is important you probably want to know about *all* of the parts the contribute to the big picture.