How is your character's size used?
Your character's size is used to determine how many squares its token takes up on a map. Normal "medium" sized creatures take up a single square. Large creatures take up 2x2 squares. The control on the character editor through which you set the character size contains the list of valid sizes. Note that these are fairly d20-centric at the moment. That seems to cover enough variants, but let me know if that's not the case. You can edit your character's size via the character editor. (See "Editing a character".)
Setting the character's size vs. resizing the character token
When you set the character's size, you're telling EpicTable how you want to snap that token's size to the grid on a map or tabletop. You can also resize a character token. However, if you do, EpicTable treats that action as taking precedence over the character's size. After you've resized a token, it will no longer be auto-sized to the grid for that map or tabletop. Essentially, EpicTable concludes that you know what you're doing when you resize a character token and don't want your resizing to be undone by a change to the map's grid.