List view will display by default.
If you do not specify a view, the contest will display the list view and grid view icons in the top right corner of the contest so that the user can switch between views.
If you would like to use grid view as your default view add view=grid to your short code like this:
showcontestants id=42 showtimer=1 showform=1 view=grid
You can hide the grid/list view selector by going to WordPress Dashboard > Contest > Contest Category > Edit Contest Category > Hide Grid/List setting.