Note that grout is not a perfect water seal. Rain WILL seep through
the grout on a table as it is a flat surface. I'd consider web articles
about constructing tile shower stalls in order to provide a permanent
piece of outdoor furniture.

Also, how you plan to finish the edges should help you determine how
thick you want the overall top surface to be. Also, bullnose tile
seems much more expensive than the basic square stuff....in my
experience anyhow.

Best of luck, sounds like a really great way to use an old table.