Foil balloons are available in sizes ranging from 4" up to 36" although 18" is generally the smallest foil that will hold enough helium to float.  An 18" foil will float for at least a week, probably more like two weeks but this depends on circumstances.  Available shapes are rounds, hearts, stars & squares although not all colours are available in all shapes, or indeed all sizes.  For a good indication of what combinations of colours, sizes and shapes see the Qualatex chart:

http://www.qualatex.com/pages/microfoil.php

The colours depicted here are for guidance only; the actual balloon colours may differ slightly.

This is just an idea of the primary colours available - see the Qualatex chart for a fuller range.