If you've got nothing better, create them in paint and save them there.
Otherwise, if you've got some form of vector drawing software (à la Adobe Illustrator, create them in there, and save them as vectors, you'll then be able to size them as you need them without loosing fidelity).
A quick google yields
"Inkscape" as an open-source attempt at making a free Adobe illustrator. Given the simplicity of what you'e aiming to achieve, it's featureset will probably be more than enough. (See Wikipedia
here for a more extensive list of free/opensource vector software).
Just make sure you save your work it in a vector not raster format !
(for the avoidance of confusion, the word vector in my case refers to "vector graphics", i.e. in contrast to "raster graphics", not special wind vector drawing software !)