You can execute a mysql query with show table status like 'myTABLE'.
The result will be a list of all table that meet the "like" criteria. (If you enter the exact name of the table, it will return a single row).
Within the returned row(s), you'll find a field named "Avg_row_length", which will give you an accurate reading.
If what you are looking for is the actual size of a specific row, you'll have to select and query that row, cycle through the array returned by MySQL and add the sizes of each field.