Lots of fun. They really should be called Creative or Artistic Journals since this is a better description of what they are.
There are no rules - make the pages any size you want. The traditional size is 5x7" but if you are using A4 papers - cut them in quarters & you will have a page size of 4x5.5" which is a great size too. Another option is to use cardstock at 5x7" & patterned papers at 4x5.5".
Add different kinds of papers to the journal - I love using vellum, either plain or tea/coffee dyed.
The traditional number of signatures is 2 BUT again - no rules. A mini journal with 1 signature can be awesome.
Styles & Colours? Whatever you want - vintage through to shabby chic; black through to white.