Hi Julie,
Not used quark 5 as i went straight to quark 6.5, but if the features are not dissimilar then there is an option to have cell boxes linked when initially creating a table. This allows you to select all text rather than just cell by cell.
From here you can only really change existing type of the font and size if you set up a character style in the edit>style sheets menu. Select the text and change the style via the contextual menu (control/right click) or the style>character style (in the main menu top of screen).
You can also change the font used in the usage window, although this is a global change and will therefore effect all the font used in the entire document (except eps of course). This won't however change the type sizes.
Hope this is of some help.
Kind regards,
Andrew