To setup the page as A5 just go into the Page Layout tab in Word and select the Size button. Choose More Paper Sizes from the drop down menu and then from the Paper size drop down select A5 148x210mm and click OK

I'd use a 2 column table to get the layout you want. Have the left hand column of the table aligned to the left and the right side aligned to the right. Turn off the borders so you get a seemless look.

Set up a style to use for your text and a different type for hers and apply a Text Highlight color to both to give the blue and green tones behind the text like in the chat format.