You can use the table.rows.length native property to get the number of the table's rows and the table.rows.item(0).cells.length native property to get the number of its columns. Please note that you need to enable the "Show hidden properties" option if you want to examine the values of these properties using the Object Browser. Please see the "Engines - General Options" help topic for more information on the option.
"Native" means something which is not added by TestComplete. For desktop applications' controls, you can find a description of their properties/methods in the controls' documentation. As for web page elements, there are many sources of information in the Internet. For example, I recommend that you take a look at the following web page: http://www.w3schools.com/default.asp