To resize the grid, hold command before dragging out to the new size. To change the size of the letters at the same time, hold down 'option' as well. To change the size of the numbers at the same time, hold down 'shift' as well.
NB The new size is based on the HEIGHT of the new window. The squares will always remain square.

The little arrow shows the direction for entering text. Click on it, or press TAB, to change the direction.

When you are in text mode, and you have selected a space by double-clicking in one, the pop-up menu above the grid will be loaded with words from the selected dictionary which will fit the space. Simply select one to enter it. The menu is updated as you change letters in the word, showing how many are available. If "Anagrams In Popup" is checked, it will show all possible anagrams instead.
NB The menu will only hold up to 255 words at a time, or 32 disabled items (a Mac limitation).