Depending on your use for the sticky note you may be best off with a specific size of sticky note.
Page marking:

If the task at hand is to mark the pages of a book or report then you should use small page mark sticky notes, it is possible to write short notes on these to remind you why you have bookmarked something and as with all stick notes are re-positionable due to their unque weak glue.
Leaving notes & reminders for yourself and colleagues:

It is always useful to have at hand both the square sticky note of 76x76mm and also the
rectangular sticky note of 76x127mm, the rectangular one is always better for holding that little bit more content.
Sticky note cubes

If you are a persistent user of sticky notes, and looking around the office I am not sure who isn't, you should consider buying a cube of post it notes. These will save you and your company money in the long run as they are economically priced.